Separate external libraries and language files into VS projects

No source files were moved, the only changes were to VS solution. The new solution structure looks like this:
![](http://puu.sh/dAaQU/3a1a167805.png)

- language project simply hold the language files. The PostBuildEvent which copies the files has been moved there.
- "libs" project holds speex, argparse and lodepng files. They are compiled to a static library (to build/libs) and linked with openrct2.dll.

Benefits:
- project - wide search will now skip language files and these libraries, which makes search results easier to navigate
- optimizations are now enabled for the libs project