shellDBus: Add UngrabAccelerators
While it is possible to register accelerators in-bulk, there is no proper way to unregister them again. This adds the corresponding call for UngrabAccelerator to allow ungrabbing multiple accelerators at the same time.
The idea is that g-s-d can use this in the future to simplify the keybinding reload logic.
NOTE: The g-s-d side is not yet implemented (I have tested the patch properly).