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. 
Oh dear fucking god that formatting, also the general concept is rather stupid. Yes we can hand hold that the screen needs to extend GuiScreen but crashing and burning is fine for me when it comes to modders being THAT stupid.