shellDBus: Add UngrabAccelerators

Benjamin Berg requested to merge benzea/accelerator-api-improvements into master

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).

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