[SlidePush] Only call stop if renderer is playing.

Previously, slidepush always called stop before pushing an image to
a DMR.  However, this is only necessary if the DMR is currently playing.
SlidePush now checks the DMR's PlayStatus to ensure it is Playing before
calling Stop.

Signed-off-by: Mark Ryan <mark.d.ryan@intel.com>
[SlidePush] SlidePush now supports PDF files.

Signed-off-by: Mark Ryan <mark.d.ryan@intel.com>