BUI - Image Filter tweaks, AllMethodsStatic, background color for containers/elements

I did not get much done since the last post. The week was pretty busy. After next weeks I will have holiday of three weeks so I expect to be more productive then. Anyway I got something done:

• I added some minor tweaks to the Image Filter example. For instance it shows the image names as labels now.
• A new base class for container and event classes, AllMethodsStatic, is available. I used some metaprogramming to define this class to mimic namespaces. Essentially inheriting from this class makes all methods of the class static. This is nice as you don't have to write @staticmethod decorators in front of each constraint/event. The solution is not particularly pythonic but I think it works in this case, at least for now.
• I made it possible to determine background color for a container or an element. This should be expanded later to make it possible to define gradient/image/whatever as a background. As a side effect implementing this clarified the code a bit. The rendering parts still need solid tests though.