Fixed exception when trying to visgroup world brushes and entities together
New settings to override the map size values for FGDs that do not specify the values
Replaced the hard-coded engine strings and ids with more friendly enums to prepare for future abstraction. Should resolve #83.
Merge branch 'VisgroupException' of https://github.com/jpiolho/sledge
Merge remote-tracking branch 'upstream/master' into Maz

Conflicts:
	Sledge.Editor/Documents/Document.cs
	Sledge.Editor/Settings/SettingsForm.Designer.cs
	Sledge.Editor/Settings/SettingsForm.cs
	Sledge.Settings/Models/Game.cs