Add a way to clear activity indicator (#30156)

Follow-up of https://github.com/zed-industries/zed/pull/30015

* Restyles the dismiss and close buttons a bit: change the dismiss icon
and add tooltips with the bindings to both
* Allows ESC to clear any status that's in the activity indicator now,
if all notifications are cleared: this won't suppress any further status
additions though, so statuses may resurface later

Release Notes:

- Added a way to clear activity indicator

crates/workspace/src/workspace.rs

@@ -869,6 +869,7 @@ pub enum Event {
     },
     ZoomChanged,
     ModalOpened,
+    ClearActivityIndicator,
 }
 
 #[derive(Debug)]
@@ -5496,6 +5497,7 @@ impl Workspace {
             return;
         }
 
+        cx.emit(Event::ClearActivityIndicator);
         cx.propagate();
     }
 }