Linux: use XDG_DATA_HOME instead of HOME for user_program_data_path

[whizikxd]

If $XDG_DATA_HOME is not set or empty, default to $HOME/.local/share, as specified in
https://specifications.freedesktop.org/basedir/latest/#variables

Related/fixes: #660

[mmozeiko]

IMHO in raddbg case these files should be split between $XDG_CONFIG_HOME (config files) and $XDG_STATE_HOME (logs/temporary files). And not use XDG_DATA_HOME at all.

In case splitting is too much, I'd prefer to use only $XDG_CONFIG_HOME instead. And fallback to ~/.config obviously.

[whizikxd]

On second thought I do agree XDG_DATA_HOME is kind of a weird place to put all the files in case of raddbg. I can either change it to XDG_CONFIG_HOME (for now atleast) or attempt to split it between the two directories which would be more involved from what i can tell.