This makes the display architecture closer to our other frontend/backend splits
Added check that display is initialised successfully in hw_init
Renamed _update_timer to just _timer to make more consistent with other drivers
Due to the way the headers are organized changing a single change in
an AP_Notify driver would trigger a rebuild for most of the files in
the project. Time could be saved by using ccache (since most of the
things didn't change) but we can do better, i.e. re-organize the headers
so we don't have to re-build everything.