Display change notifications

Hey all:

This is a set of commits that hooks into OS notifications for display connect and disconnect.  I needed this so I could display a full-screen "performance window" for any secondary displays that were connected to my laptop.  Its one less thing to think about while at shows where I'm getting disconnected from and reconnected to the projector at the booth without my knowledge.. ;) 

It looks like this work was started at some point then stopped, so I'm not sure if there was a reason beyond time constraints.   I've only really tested this properly on OSX and iOS.  I don't have a good multimonitor setup for Win32 at the moment.