volumeosd: Properly bind volume keys

Partially addresses plasma/plasma-mobile#328

Apparently in in Qt 6 the "shortcut" property in QAction no longer maps properly to the given Qt key??

Qt.Key_VolumeUp/Qt.Key_VolumeDown -> no key

Then I tried putting in Qt.Key_A which mapped to Qt.Key_Ctrl + Qt.Key_Q... Using the `shortcuts` property made it work properly here.

This will be obsolete with Plasma 6.1 plasma/plasma-pa!221