applets/battery: support releasing app inhibitions

Motivation: see bug report

How to test:

1. Build this together with the associated powerdevil MR
2. Restart powerdevil: pkill org_kde_powerde -u $UID; ~/kde/usr/lib/libexec/org_kde_powerdevil & and the Plasma applet
3. Play a video and wait five seconds for the inhibition to appear
4. Press the button
5. Confirm computer automatically goes to sleep after the inactivity period set in power management settings even while video is still playing

BUG: 423745

Depends on powerdevil!353 (closed)