Add centered layout support (#9754)

This PR implements the Centered Layout feature (#4685):
- Added the `toggle centered layout` action.
- The centered layout mode only takes effect when there's a single
central pane.
- The state of the centered layout toggle is saved / restored between
Zed restarts.
- The paddings are controlled by the `centered_layout` setting:

```json
"centered_layout": {
  "left_padding": 0.2,
  "right_padding": 0.2
}
```

This allows us to support both the VSCode-style (equal paddings) and
IntelliJ-style (only left padding in Zen mode).

Release Notes:

- Added support for Centered Layout
([#4685](https://github.com/zed-industries/zed/pull/9754)).


https://github.com/zed-industries/zed/assets/2101250/2d5b2a16-c248-48b5-9e8c-6f1219619398

Related Issues:

- Part of #4382

crates/workspace/src/persistence.rs

@@ -138,6 +138,7 @@ define_connection! {
     //   window_height: Option<f32>, // WindowBounds::Fixed RectF height
     //   display: Option<Uuid>, // Display id
     //   fullscreen: Option<bool>, // Is the window fullscreen?
+    //   centered_layout: Option<bool>, // Is the Centered Layout mode activated?
     // )
     //
     // pane_groups(
@@ -280,6 +281,10 @@ define_connection! {
     sql!(
         ALTER TABLE workspaces ADD COLUMN fullscreen INTEGER; //bool
     ),
+    // Add centered_layout field to workspace
+    sql!(
+        ALTER TABLE workspaces ADD COLUMN centered_layout INTEGER; //bool
+    ),
     // Add preview field to items
     sql!(
         ALTER TABLE items ADD COLUMN preview INTEGER; //bool
@@ -299,12 +304,13 @@ impl WorkspaceDb {
 
         // Note that we re-assign the workspace_id here in case it's empty
         // and we've grabbed the most recent workspace
-        let (workspace_id, workspace_location, bounds, display, fullscreen, docks): (
+        let (workspace_id, workspace_location, bounds, display, fullscreen, centered_layout, docks): (
             WorkspaceId,
             WorkspaceLocation,
             Option<SerializedWindowsBounds>,
             Option<Uuid>,
             Option<bool>,
+            Option<bool>,
             DockStructure,
         ) = self
             .select_row_bound(sql! {
@@ -318,6 +324,7 @@ impl WorkspaceDb {
                     window_height,
                     display,
                     fullscreen,
+                    centered_layout,
                     left_dock_visible,
                     left_dock_active_panel,
                     left_dock_zoom,
@@ -344,6 +351,7 @@ impl WorkspaceDb {
                 .log_err()?,
             bounds: bounds.map(|bounds| bounds.0),
             fullscreen: fullscreen.unwrap_or(false),
+            centered_layout: centered_layout.unwrap_or(false),
             display,
             docks,
         })
@@ -678,6 +686,14 @@ impl WorkspaceDb {
             WHERE workspace_id = ?1
         }
     }
+
+    query! {
+        pub(crate) async fn set_centered_layout(workspace_id: WorkspaceId, centered_layout: bool) -> Result<()> {
+            UPDATE workspaces
+            SET centered_layout = ?2
+            WHERE workspace_id = ?1
+        }
+    }
 }
 
 #[cfg(test)]
@@ -764,6 +780,7 @@ mod tests {
             display: Default::default(),
             docks: Default::default(),
             fullscreen: false,
+            centered_layout: false,
         };
 
         let workspace_2 = SerializedWorkspace {
@@ -774,6 +791,7 @@ mod tests {
             display: Default::default(),
             docks: Default::default(),
             fullscreen: false,
+            centered_layout: false,
         };
 
         db.save_workspace(workspace_1.clone()).await;
@@ -873,6 +891,7 @@ mod tests {
             display: Default::default(),
             docks: Default::default(),
             fullscreen: false,
+            centered_layout: false,
         };
 
         db.save_workspace(workspace.clone()).await;
@@ -902,6 +921,7 @@ mod tests {
             display: Default::default(),
             docks: Default::default(),
             fullscreen: false,
+            centered_layout: false,
         };
 
         let mut workspace_2 = SerializedWorkspace {
@@ -912,6 +932,7 @@ mod tests {
             display: Default::default(),
             docks: Default::default(),
             fullscreen: false,
+            centered_layout: false,
         };
 
         db.save_workspace(workspace_1.clone()).await;
@@ -949,6 +970,7 @@ mod tests {
             display: Default::default(),
             docks: Default::default(),
             fullscreen: false,
+            centered_layout: false,
         };
 
         db.save_workspace(workspace_3.clone()).await;
@@ -983,6 +1005,7 @@ mod tests {
             display: Default::default(),
             docks: Default::default(),
             fullscreen: false,
+            centered_layout: false,
         }
     }