Add basic framework to retrieve font colors from fallback settings

Used only by Button and SandText skins so far. Want to make sure that the framework is solid before doing the rest. I had to pass the fallbackMap to the WindowManager, getting it from World is not possible because GUI is loaded before the world even exists.
I think the idea was to move some fallback settings to the content files later, however that is not possible for the font colors. Fonts are already shown in the loading screen before all content files are loaded.
This is not really a problem, as MyGUI is flexible enough to have such changes in a simple resource file listing modified tags. All we need to do is bundle such a resource file with a mod. Anyway, this is post 1.0 territory again, so might as well worry about it later.