Add video support to legacy map importer

This enables the map importer to map the .ini video definitions to ours.

The mapping generally is as follows:

Intro => BackgroundInfo
Brief => Briefing
Action => GameStart
Win => GameWon
Lose => GameLost

An issue in some Red Alert maps means that this mapping is not always
quite correct. In those maps that do not have a 'Brief' video defined
(scg03a is an example), Westwood has assigned the video that should
probaby have been the 'Action' video to the 'Intro' slot instead. I can
only assume that that was done due to some limitation in the original
game code. Mappers will have to correct that assignment manually in
those cases.