Added initial support for loading user skins.
- "Skins" directory is created in the root folder, and reads files directly from there (for now).  NOT ALL FILES ARE SUPPORTED.
- To change the path (e.g. to an installed skin), edit the path in the configuration file.

Other changes:
- Renamed some files: directories now begin with a capital letter (to match osu! naming), and '.osu_tmp' renamed to '.opsu_tmp'.
- Disabled mouse grabbing, as this does not seem to work properly for touchscreen devices. (partial revert of ab487c5)

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>