WinRT: Made app-backgrounded events get sent at separate, distinct times.

SDL_APP_WILLENTERBACKGROUND is now sent as soon as the app is told that it is
about to go to the background.  SDL_APP_DIDENTERBACKGROUND is sent via a WinRT
'deferral operation', which is how WinRT gives apps a bit of extra time
(multiple seconds worth) to prepare for an app-backgrounding.

The distinction may be important as the deferral operation's code is always run
in a separate thread.  For Direct3D-only apps, this means that between the
two SDL app-backgrounded events, SDL_APP_WILLENTERBACKGROUND will be the only
one run from the main thread.  Given that some WinRT operations can only be done
on the main thread (operations to the CoreWindow fall into this category), this
could be important.

It is important to note that pre-deferral code may only have a very short bit of
time to execute code, less so than code run in the deferral operation (where
SDL_APP_DIDENTERBACKGROUND is sent from), which usually gets several seconds to
run.