GuiConfig instance fixes

Simple fix that changes the mainConfigGuiClass() method in IModGuiFactory to be able to use instances. The advantages being:

-Less overhead (no need to instantiate via reflection)
-More flexible (the instances can be instantiated however the user wishes)

I also made it so the modder can still return a class object if they so wish, however, it is a bit redundant. 
Cuz Screw backwards compatibility!