use xdg directory spec for config files

Quoting original submitter at https://bugs.debian.org/696851:

> launchy currently saves its config file (launchy.ini) and database
> (launchy.db) in the users home dir not even bothering to save them as
> dot-files. This is not nice.
> 
> The best thing IMHO would be to conform to the XDG directory specification[1].
> 
> [1] http://wiki.debian.org/XDGBaseDirectorySpecification

Thanks,
