Prevent kglobalaccel5 getting activated on non-Plasma systems

While in theory kglobalaccel works on any X11 system we don't want it to run on non-Plasma systems. It isn't very useful and may interfere with the desktop's native shortcut system.

Calling some API of KGlobalAccel may result in a DBus call to the daemon which then may launch by DBus activation. Prevent that from happening on non-Plasma systems.

BUG: 435420
CCBUG: 430691