Add preview tabs (#9125)

This PR implements the preview tabs feature from VSCode.
More details and thanks for the head start of the implementation here
#6782.

Here is what I have observed from using the vscode implementation ([x]
-> already implemented):
- [x] Single click on project file opens tab as preview
- [x] Double click on item in project panel opens tab as permanent
- [x] Double click on the tab makes it permanent
- [x] Navigating away from the tab makes the tab permanent and the new
tab is shown as preview (e.g. GoToReference)
- [x] Existing preview tab is reused when opening a new tab
- [x] Dragging tab to the same/another panel makes the tab permanent
- [x] Opening a tab from the file finder makes the tab permanent
- [x] Editing a preview tab will make the tab permanent
- [x] Using the space key in the project panel opens the tab as preview
- [x] Handle navigation history correctly (restore a preview tab as
preview as well)
- [x] Restore preview tabs after restarting
- [x] Support opening files from file finder in preview mode (vscode:
"Enable Preview From Quick Open")
 
I need to do some more testing of the vscode implementation, there might
be other behaviors/workflows which im not aware of that open an item as
preview/make them permanent.

Showcase:


https://github.com/zed-industries/zed/assets/53836821/9be16515-c740-4905-bea1-88871112ef86


TODOs
- [x] Provide `enable_preview_tabs` setting
- [x] Write some tests
- [x] How should we handle this in collaboration mode (have not tested
the behavior so far)
- [x] Keyboard driven usage (probably need workspace commands)
- [x] Register `TogglePreviewTab` only when setting enabled?
- [x] Render preview tabs in tab switcher as italic
- [x] Render preview tabs in image viewer as italic
- [x] Should this be enabled by default (it is the default behavior in
VSCode)?
- [x] Docs

Future improvements (out of scope for now):
- Support preview mode for find all references and possibly other
multibuffers (VSCode: "Enable Preview From Code Navigation")


Release Notes:

- Added preview tabs
([#4922](https://github.com/zed-industries/zed/issues/4922)).

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/workspace/src/persistence/model.rs

@@ -246,6 +246,7 @@ impl SerializedPane {
     ) -> Result<Vec<Option<Box<dyn ItemHandle>>>> {
         let mut item_tasks = Vec::new();
         let mut active_item_index = None;
+        let mut preview_item_index = None;
         for (index, item) in self.children.iter().enumerate() {
             let project = project.clone();
             item_tasks.push(pane.update(cx, |_, cx| {
@@ -261,6 +262,9 @@ impl SerializedPane {
             if item.active {
                 active_item_index = Some(index);
             }
+            if item.preview {
+                preview_item_index = Some(index);
+            }
         }
 
         let mut items = Vec::new();
@@ -281,6 +285,14 @@ impl SerializedPane {
             })?;
         }
 
+        if let Some(preview_item_index) = preview_item_index {
+            pane.update(cx, |pane, cx| {
+                if let Some(item) = pane.item_for_index(preview_item_index) {
+                    pane.set_preview_item_id(Some(item.item_id()), cx);
+                }
+            })?;
+        }
+
         anyhow::Ok(items)
     }
 }
@@ -294,14 +306,16 @@ pub struct SerializedItem {
     pub kind: Arc<str>,
     pub item_id: ItemId,
     pub active: bool,
+    pub preview: bool,
 }
 
 impl SerializedItem {
-    pub fn new(kind: impl AsRef<str>, item_id: ItemId, active: bool) -> Self {
+    pub fn new(kind: impl AsRef<str>, item_id: ItemId, active: bool, preview: bool) -> Self {
         Self {
             kind: Arc::from(kind.as_ref()),
             item_id,
             active,
+            preview,
         }
     }
 }
@@ -313,20 +327,22 @@ impl Default for SerializedItem {
             kind: Arc::from("Terminal"),
             item_id: 100000,
             active: false,
+            preview: false,
         }
     }
 }
 
 impl StaticColumnCount for SerializedItem {
     fn column_count() -> usize {
-        3
+        4
     }
 }
 impl Bind for &SerializedItem {
     fn bind(&self, statement: &Statement, start_index: i32) -> Result<i32> {
         let next_index = statement.bind(&self.kind, start_index)?;
         let next_index = statement.bind(&self.item_id, next_index)?;
-        statement.bind(&self.active, next_index)
+        let next_index = statement.bind(&self.active, next_index)?;
+        statement.bind(&self.preview, next_index)
     }
 }
 
@@ -335,11 +351,13 @@ impl Column for SerializedItem {
         let (kind, next_index) = Arc::<str>::column(statement, start_index)?;
         let (item_id, next_index) = ItemId::column(statement, next_index)?;
         let (active, next_index) = bool::column(statement, next_index)?;
+        let (preview, next_index) = bool::column(statement, next_index)?;
         Ok((
             SerializedItem {
                 kind,
                 item_id,
                 active,
+                preview,
             },
             next_index,
         ))