Add classic left-click orders

This adds the most frequently found wish on Facebook and ModDB. It is based on the HotkeyAddon branch by @TiriliPiitPiit (reversed commit 9a655d1a46fc631b5fe27074acdab00c2d6e1717 and http://dpaste.com/874285/) as well as https://github.com/Iran/ClassicRA/blob/master/OpenRA.Game/Widgets/WorldInteractionControllerWidget.cs by @Iran with some changes to fix known issues and keep it optional. More tricky than expected.