vlcmediawidget: workaround a bug at pause status report

libVlc were supposed to report libvlc_Paused when status changes
to pause. However, the first time pause is pressed, it reports,
instead, libvlc_Playing. So, instead of relying on it, let's
internally cache when the MediaWidget requests to set pause, and
use the internal cache to properly report it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>