Audio: Add mute/unmute and volume shortcuts

[Megamouse]

• Add mute/unmute shortcut
• Add volume shortcuts with auto repeat

fixes #13428

[wxvu]

The shortcuts added in this PR conflict with normal keyboard usage inside RPCS3 and are always active, unaffected by the 'Disable keyboard hotkeys' setting.

This feels like an oversight. Using the basic keyboard handler, it is now impossible to type certain keys (M and _), triggering these shortcuts instead. Currently the only available solution is to edit the default shortcut hotkeys globally.

[wxvu]

The default shortcut hotkey for Volume Up is impossible to trigger as well.

#16664 seems like a good fix to some of the issues mentioned above, but would be possible to implement a per-game option for disabling keyboard hotkeys too? Seeing as enabling the basic keyboard handler itself is also able to be set per-game. Would be very beneficial I feel, can create a Feature Request for it as to not clog here any further.