Closes #61005

The bookmark store notified on every change, but nothing was listening.
Editors only repainted their bookmark icons because the toggle actions
happened to notify the editor directly - so workspace::ClearBookmarks,
which never touches an editor, left stale icons in the gutter until
something else caused a repaint.

Now editors observe the bookmark store, so any change repaints them.
This also fixes toggling a bookmark in one split not updating another
split showing the same file.

Release Notes:

- Fixed bookmarks not disappearing from the gutter immediately when
running `workspace::ClearBookmarks`.