WIP: Add a Shift + Volume Mute hotkey that mutes only the default (selected) device

This is to address https://bugs.kde.org/show_bug.cgi?id=434217

Currently the default mute key behavior mutes all devices on a system. This does not align with other operating systems and is inconsistent with the mute behavior for the microphones. This patch adds a single device mute function and associates the mute a Shift + Volume Mute shortcut to it. It also adds a new GlobalAction so that users can configure a global mute shortcut if they so choose.

This patch is WIP as there is more things to address in the bug:

1.) I am not sure what the right way is to add a QKeySequence in QML. I want the default global mute shortcut to be Shift + Volume Mute, but currently I am only able to assign single keys. If anyone can help me with that it would be much appreciated.

2.) The bug report also addresses exposing a global mute icon in the widget. Currently there is actually a global mute icon, but it is hidden by default. I don't want to unhide it without understanding what the reasoning behind hiding it in the first place was.

3.) I want to fix the global mute behavior a bit more. Currently, on my system, sometimes devices will unmute themselves when attempting a global mute. In particular, Digital Output (S/PDIF Starship/Matisse HD Audio Controller) loves to unmute right after a global mute. When I attempt to global unmute, it fails to unmute all muted devices because the list of globally muted devices doesn't line up with what the plasma-pa widget is aware of and so it only does a single device unmute. This will be done in a separate patch.