Support XDG base directory specification under Linux

I've noticed that the Linux version of BuildGDX keeps save, configuration, and log files in ~/M210Projects.

Could you support the XDG base directory specification by using XDG_DATA_HOME (~/.local/share by default) and $XDG_CONFIG_HOME (~/.config by default) instead?

(You can read more about the specification here and here.)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information