Move data and templates to userdata subdirs

[ghisvail]

No description provided.

[ghisvail]

Assuming #133 is merged, that means data would end up under ${XDG_DATA_HOME}/higan-ui/Systems or ~/.local/share/higan-ui/Systems and templates under ${XDG_DATA_HOME}/higan-ui/Templates or ~/.local/share/higan-ui/Templates.

Path were chosen to remain consistent with the sample path.bml and avoid polluting $HOME with yet another application directory, which is what the XDG Base Directory Specification is here to avoid.

[ghisvail]

This PR probably is wrong as I don't get the point of the templates directory in this setup. I guess templates should point to (prefix)/share/higan where the content of the System folder is installed, correct?

[ghisvail]

Ok, I have looked at the packaging for Arch and Debian and both handle these paths in different ways:

• On Arch, templates are installed to /usr/share/higan/Templates and data is set to $HOME/.local/higan/Systems
• On Debian, templates are installed to /usr/share/higan and data is set to $HOME/.local/higan

The question is whether the data directory is supposed to evolve with more content, or is it supposed to only host systems defined by the user and nothing more. For the latter, the Debian way is fine. The Arch way is more flexible if desired.