Enable more collab UI features (#3496)

* Current Call section of the collab panel
* Improve the collab titlebar
* Add basic UI for following

Following only partially works, but the UI for following is now in
place.

crates/gpui2/src/window.rs

@@ -2734,6 +2734,7 @@ pub enum ElementId {
     Integer(usize),
     Name(SharedString),
     FocusHandle(FocusId),
+    NamedInteger(SharedString, usize),
 }
 
 impl ElementId {
@@ -2783,3 +2784,9 @@ impl<'a> From<&'a FocusHandle> for ElementId {
         ElementId::FocusHandle(handle.id)
     }
 }
+
+impl From<(&'static str, EntityId)> for ElementId {
+    fn from((name, id): (&'static str, EntityId)) -> Self {
+        ElementId::NamedInteger(name.into(), id.as_u64() as usize)
+    }
+}