Use handles to obtain entities in future callbacks

This guarantees that the spawning entity will be present and simplifies
the logic for obtaining the entity. Now we can forward the results of spawned futures and streams in the same way regardless of whether what spawned them was a model or a view.

zed/src/editor/buffer_view.rs

@@ -138,7 +138,7 @@ impl BufferView {
 
         let buffer_ref = buffer.as_ref(ctx);
         Self {
-            handle: ctx.handle(),
+            handle: ctx.handle().downgrade(),
             buffer,
             display_map,
             selections: vec![Selection {

zed/src/file_finder.rs

@@ -275,7 +275,7 @@ impl FileFinder {
         settings.notify_view_on_change(ctx);
 
         Self {
-            handle: ctx.handle(),
+            handle: ctx.handle().downgrade(),
             settings,
             workspace,
             query_buffer,