applets/systemtray: Support Plasmoid.busy property in PlasmoidItem wrapper

This brings feature-parity with desktop package and panel containment to system tray: plasmoid won't need to implement custom busy indicators on their own in their compact representations components.

CC @teams/vdg

Labels are both feature and a bug fix, as this is a feature which was supposed to be there from the beginning, but probably due to an omission was ignored until now.