Fix item closing overly triggering save dialogues (#21374)

Closes https://github.com/zed-industries/zed/issues/12029

Allows to introspect project items inside items more deeply, checking
them for being dirty.
For that:
* renames `project::Item` into `project::ProjectItem`
* adds an `is_dirty(&self) -> bool` method to the renamed trait
* changes the closing logic to only care about dirty project items when
checking for save prompts conditions
* save prompts are raised only if the item is singleton without a
project path; or if the item has dirty project items that are not open
elsewhere

Release Notes:

- Fixed item closing overly triggering save dialogues

crates/project/src/image_store.rs

@@ -1,6 +1,6 @@
 use crate::{
     worktree_store::{WorktreeStore, WorktreeStoreEvent},
-    Project, ProjectEntryId, ProjectPath,
+    Project, ProjectEntryId, ProjectItem, ProjectPath,
 };
 use anyhow::{Context as _, Result};
 use collections::{hash_map, HashMap, HashSet};
@@ -114,7 +114,7 @@ impl ImageItem {
     }
 }
 
-impl crate::Item for ImageItem {
+impl ProjectItem for ImageItem {
     fn try_open(
         project: &Model<Project>,
         path: &ProjectPath,
@@ -151,6 +151,10 @@ impl crate::Item for ImageItem {
     fn project_path(&self, cx: &AppContext) -> Option<ProjectPath> {
         Some(self.project_path(cx).clone())
     }
+
+    fn is_dirty(&self) -> bool {
+        false
+    }
 }
 
 trait ImageStoreImpl {