WinRT: Miscellaneous app-backgrounding/restoring event fixes and additions

SDL_WINDOWEVENT_FOCUS_LOST is now sent when an app's native window is hidden.
Likewise, SDL_WINDOWEVENT_FOCUS_GAINED is sent when an app's window is shown.
This mimicks behavior seen on iOS and Android.

SDL_WINDOWEVENT_MINIMIZED and SDL_WINDOWEVENT_RESTORED are now sent when the
app's native window is hidden and shown.  Previously, these were sent when an
app was suspended and resumed.  On Windows 8.x/RT, an app may be sent to the
background without being suspended, which previously meant that
SDL_WINDOWEVENT_MINIMIZED might never have been sent.  (On Windows Phone 8,
however, this seems to be different, whereby apps sent to the background appear
to always get suspended.)