Move FMV playback from Lua scripts to widgets

showVideoPlayer, infoVideoPlaying, briefingVideoPlaying, actionVideoPlaying, videoPlaying can be replaced by an enum and a single field or property.
```C#
enum PlayingMissionVideo { None, Info, Briefing, Action }
...
	PlayingMissionVideo PlayingVideo;
	# Substitutions below - not actual code:
	bool infoVideoPlaying { get { return PlayingVideo == PlayingMissionVideo.Info; } }
	bool briefingVideoPlaying { get { return PlayingVideo == PlayingMissionVideo.Briefing; } }
	bool actionVideoPlaying { get { return PlayingVideo == PlayingMissionVideo.Action; } }
	bool videoPlaying { get { return PlayingVideo != PlayingMissionVideo.None; } }
```