messageTray: Add removeAction() method and allow references to actions

messageTray: Add removeAction() method and allow references to actions

Internally in gnome-shell it may be useful to modify the actions
available on a notification while it’s still visible, but without
clearing them all and re-creating them.

Allow that by returning the action object when adding it, and adding a
`removeAction()` method which takes that object and removes it.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3251>