Lexical: Updated dropdown handling to match tinymce behaviour

Now toolbars stay open on mouse-out, and close on other toolbar open,
outside click or an accepted action.
To support:
- Added new system to track and manage open dropdowns.
- Added way for buttons to optionally emit events upon actions.
- Added way to listen for events.
- Used the above to control when dropdowns should hide on action, since
  some dont (like overflow containers and split dropdown buttons).