Fix a bug in GtkActionMuxer when widgets are reparented

Previously, unparenting a widget would cause all actions from parents to be removed from the widget's ActionMuxer. If another action with the same name existed in a child of the unparented widget, that action would also be removed.

Fixed by checking whether the muxer has its own action by the same name, and not removing it if so.

Fixes #2398.