GtkShortcutManager allows adding controllers to it. For the default
implementation, they get added to one of two models, based on the
propagation phase (either GTK_PHASE_CAPTURE or GTK_PHASE_BUBBLE).

However, when a controller is removed, its presence in the manager gets
checked against the current propagation phase of the controller, which
may have changed from when it was added. This can lead to crashes if the
controller was not disposed properly since it still has a reference in
one of the two models of the GtkShortcutManager.

To fix this, add a callback for `notify::propagation-phase`, which
removes the controller from all possible models and readds it again with
its current phase. This callback is only disconnected permanently when
the controller is manually removed with
`gtk_shortcut_manager_default_remove_controller()`.

Closes #6246