dataengines/powermanagement: set result for inhibition request, and only issue one inhibition at a time
First commit: set result for inhibition request. This is needed for the battery and display configuration applets to update state (reenable switch and show OSD or error notification).
Second commit: don't trigger another inhibition request while one is already in place. Otherwise, when triggering the inhibition from the applet again while another one is still on hold, the applet will forget the cookie for the first inhibition and be unable to release it again.
Also needs kscreen!262 (merged).
Edited by Natalie Clarius