Allow to set phone to silent mode when pressing vol- on incoming calls

This sets the phone to silent mode on incoming calls when vol- is pressed and the device is locked.

It needs https://source.puri.sm/Librem5/feedbackd/-/merge_requests/128 and GNOME/calls!734 (merged) hence draft for the moment.

As this might not be desirable for everyone we currently hide it behind an experimental gsetting (which will get a toggle in mobile settings)

@kop3161 I think you had an issue filed requesting that a while back but I can't seem to find it.

Closes: #780 (closed)