Resources/Assets: Clean up and load in a consistent way #81

Two things bother me a bit:

• The TextManager is a total mess (decompiled) and loading the XML files in totally different ways than the other resources (f.e. using HTTP)
• Most of the resources is actually unused or there are 3 versions of some Minigolf textures at different places. Let's reduce to an absolute minimum. Assets from other games can be still found via the legacy branch if required.