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/diagnostics/src/diagnostics.rs

@@ -501,7 +501,12 @@ impl ProjectDiagnosticsEditor {
 }
 
 impl workspace::Item for ProjectDiagnosticsEditor {
-    fn tab_content(&self, style: &theme::Tab, cx: &AppContext) -> ElementBox {
+    fn tab_content(
+        &self,
+        _detail: Option<usize>,
+        style: &theme::Tab,
+        cx: &AppContext,
+    ) -> ElementBox {
         render_summary(
             &self.summary,
             &style.label.text,