Use default output device when changing global volume

Global shortcuts now changes volume of default output device
only, instead of all output devices. Applet icon now displays
volume of default output device only, instead of first output device
in list.

Changing volume with wheel and muting with middle click on applet icon
no longer shows OSD.

Added tooltip subtext for applet showing volume level and name of
default output device.

BUG: 352370
BUG: 357214
BUG: 352727

Differential Revision: https://phabricator.kde.org/D1289