backend: Reset idletime when unplugging the power cable
Avoid having laptops suspend or lock as soon as the power cable is unplugged as the timeout for those actions when on battery are smaller than the timeouts when on AC.
- laptop is plugged in, and hasn't been used for X minutes
- laptop is unplugged
- the gnome-settings-daemon power plugin sets up its timeouts for inactivity for the "on battery" case
- those X minutes of inactivity are still counting, and are above the level of one of the timeouts (say, suspend or lock screen), mutter fires the timeouts
- gsd-power activates the action associated with the timeout
Fixes: #1953 (closed)