[win32] Remove Semaphore and ReadWriteLock classes

Both are currently unused and could be reimplemented in a platform independent way using the C++11 thread support library.
[win32] Implement OS dependent search paths

- Added new namespace umve
- Implemented umve::get_search_paths() which pushes OS dependent search paths into a given vector
- Use umve::get_search_paths() for plugins and shaders