media-keys: Suppress power-button presses during suspend

On some devices the power-button sends a KEY_POWER event immediately
after resume when the power-button was used to wakeup the system, causing
the system to suspend immediately again. This is esp. a problem on tablets
where the power-button is often the only way to wakeup the system.

This commit fixes this by listening for suspend/resume signals from
logind (which requires taking a delay inhibitor on suspend to do reliabe)
and on suspend temporarily disabling the power-button.

https://bugzilla.gnome.org/show_bug.cgi?id=789771