Add manual/"hold" option to audio control groups

[pnwparksfan]

Currently hold control can be specified for individual modes within a control group. This allows for manual siren functionality (press and hold a key, and the siren sounds until it is released). Adding a hold control to the audio control group itself would allow for more sophisticated behavior.

• If the group is currently not enabled, then pressing hold on the group should temporarily activate the mode that was most recently selected in the group
• If the group is currently enabled, pressing hold on the group should temporarily activate mode i+1 (with looping) of the most recently selected in the group

This would allow for e.g. having the group in wail mode (on) and manually yelping while pressing the hold button, or having the group off and manually activating the most recent mode.