Rename MythUINotificationCenter into MythNotificationCenter

This is to make naming consisten with other myth libraries, and using MythUI prefix only for MythUI widgets.
Make distclean required
Make mythnotificationcenter.h included in mythmainwindow.h

Make consistent use of GetNotificationCenter() across the board as this is the official way of getting the Notification Center instance. This allows to remove the need to include mythnotificationcenter.h in most cases.
Way too many includes already...