Lua API : Added floating text and radar video

This adds two new functions to the Media lua api.
1. FloatingText (string text, WPos position, int duration = 30) displays a text at the specified location.
2. LoadMovieAsync(string movie, LuaFunction func) and 
    PlayMovieInRadar(VqaReader reader, LuaFunction func = null) plays a video over the ingame radar which is used in some CnC campaigns

The simplest way to use the latter is Media.LoadMovieAsync("[videoName].vqa", Media.PlayMovieInRadar), but lua callbacks [func(VqaReader video)] are supported as well as storing the video in a global lua variable.

The OpenRA code style is to add usings instead of qualifying the type names (occurs several times in this pr).