Api to know whether an applet supportslaunchers

Right now when an app is already pinned to the taskmanager, kickoff
thinks as is not pinnable anymore, adding the icon widget to the
panel is still needed.

This removes both the pin and the add icon to panel actions when is already pinned

On the long term i would like to have the same per activity
context menu that the taskbar itself has, but since it would
be a more invasive change, should be at least for 6.1

Depends on plasma-workspace!3819 or plasma-workspace!3826

BUG: 480198
FIXED-IN: 6.0