Proposed changes editor features (#18373)

This PR adds some more functionality to the Proposed Changes Editor
view, which we'll be using in
https://github.com/zed-industries/zed/pull/18240 for allowing the
assistant to propose changes to a set of buffers.

* Add an `Apply All` button, and fully implement applying of changes to
the base buffer
* Make the proposed changes editor searchable
* Fix a bug in branch buffers' diff state management

Release Notes:

- N/A

crates/zed/src/zed.rs

@@ -14,6 +14,7 @@ use breadcrumbs::Breadcrumbs;
 use client::ZED_URL_SCHEME;
 use collections::VecDeque;
 use command_palette_hooks::CommandPaletteFilter;
+use editor::ProposedChangesEditorToolbar;
 use editor::{scroll::Autoscroll, Editor, MultiBuffer};
 use feature_flags::FeatureFlagAppExt;
 use gpui::{
@@ -582,6 +583,8 @@ fn initialize_pane(workspace: &mut Workspace, pane: &View<Pane>, cx: &mut ViewCo
             let buffer_search_bar = cx.new_view(search::BufferSearchBar::new);
             toolbar.add_item(buffer_search_bar.clone(), cx);
 
+            let proposed_change_bar = cx.new_view(|_| ProposedChangesEditorToolbar::new());
+            toolbar.add_item(proposed_change_bar, cx);
             let quick_action_bar =
                 cx.new_view(|cx| QuickActionBar::new(buffer_search_bar, workspace, cx));
             toolbar.add_item(quick_action_bar, cx);