Fix videos from being completely unrecognised by picture slideshow.

This allows videos to be played in a picture slideshow, if the user has video extensions also enabled for their picture library.

Currently, even though there is code to play videos, they do not play as slideshow items do not have a mimetype or VideoInfoTag. This results in a black screen when a video is encountered.

Issues that have also been resolved are:
- correct transitions to/from the original window to WINDOW_SLIDESHOW to WINDOW_FULLSCREEN_VIDEO, etc.
- issues of loading videos on top of each other, if there are only videos in the slideshow
- preventing background loaded images from showing between two consecutive videos
- playing of videos in non-"slideshow" mode (i.e. just selecting a picture from the list then moving back or forth to a video elsewhere in the list).