[PVR] Feature: Confirm shutdown if PVR backend is not idle.

User story: In case shutdown is requested by the user while PVR is recordig or a  recording will start very soon (and XBMC machine potentially cannot restart fast enough), XBMC refuses to shutdown, but shows a Yes/No dialog to the user explaining that PVR is currently not idle and asking the user to shut down anyway. If user does nothing for 10 secs, the dialog will autoclose and shutdown will be cancelled.

Purpose: This is very useful to prevent accidentally stopped recordings.

BTW: This feature has been discussed from time to time in the forums and there were some (hacky) addons around to implement this, but no clean implementation (which IMO can only be done inXBMC itself.)
we need an option to disable this feature for users with a setup where this does harm.