Differentiate among tabs with the same name

This commit introduces a new, optional `Item::tab_description` method
that lets implementers define a description for the tab with a certain
`detail`. When two or more tabs match the same description, we will
increase the `detail` until tabs don't match anymore or increasing the
`detail` doesn't disambiguate tabs any further.

As soon as we find a valid `detail` that disambiguates tabs enough, we
will pass it to `Item::tab_content`. In `Editor`, this is implemented by
showing more and more of the path's suffix as `detail` is increased.

crates/editor/src/editor.rs

@@ -1073,7 +1073,7 @@ impl Editor {
         &self.buffer
     }
 
-    pub fn title(&self, cx: &AppContext) -> String {
+    pub fn title<'a>(&self, cx: &'a AppContext) -> Cow<'a, str> {
         self.buffer().read(cx).title(cx)
     }