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

[PhilippvK]

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.

[StenAL]

I did some clean up for TextManager resource loading in #125 (removing dead code to load .loc files, removing code duplication, simplifying the logic to load files) and further simplified the logic in d01af74 so a lot of the work is done.

There's still some more improvements to do here like removing the custom XML parsing logic and using Java standard library APIs instead.