Adhere to XDG Base Directory Specification

[fenuks]

Ultrastar uses ~/.WorldParty to store its data on Unix platforms. In order to declutter home directory, decouple configuration from other kinds of data, and allow users to configure where files are stored in easy manner. XDG Base Directories standard has been introduced. Software compliant with it should use various XDG_* which allow user to specify where files should be stored.

I think .WorldParty directory should be split in following fashion:

• config.ini goes to $XDG_CONFIG_HOME/WorldParty (if not defined XDG_CONFIG_HOME defaults to ~/.config
• logs directory goes to $XDG_CACHE_HOME/WorldParty (if not defined XDG_CACHE_HOME defaults to ~/.cache
• rest of files to $XDG_DATA_HOME/WorldParty (if not defined, XDG_DATA_HOME defaults to ~/.local/share/)

I'm not entirely convinced that WorldParty is best name on Unix, because of casing. Perhaps directory could be named ultrastarwp instead?

Also see Debian Wiki for table showing what each directory should contain (at the very bottom of the page)

In order to not disrupt existing users, if ~/.WorldParty already exists, it should be used instead.