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.

OpenRA.Game/Map/Map.cs

@@ -408,6 +408,8 @@ namespace OpenRA
 				root.Add(new MiniYamlNode(field, FieldSaver.FormatValue(this, f)));
 			}
 
+			root.Add(new MiniYamlNode("Videos", FieldSaver.SaveDifferences(Videos, new MapVideos())));
+
 			root.Add(new MiniYamlNode("Options", FieldSaver.SaveDifferences(Options, new MapOptions())));
 
 			root.Add(new MiniYamlNode("Players", null,