Add resetIdleSeconds() to the xf86misc module and implement it for powermanager.

This function is needed to simulate activity events.
When the machine suspends after being a number of seconds idle, it would not reset this idle time post resume. The next time, the idle time is checked, it's higher than the configured value again, and the machine goes to sleep immediately again.

The following events now reset the idle time:
- unplugging the AC adapter
- plugging in the AC adapter
- waking up from suspend
- waking up from hibernate