Use the id property of PurpleUiInfo as the directory name to use with XDG_CONFIG_HOME

Testing Done:
Ran both finch and pidgin.
Verified that they created `build/config/finch3` and `build/config/pidgin3` when run from a devenv environment.
Also verified that `gtk-gaim.ini` in `build/config/pidgin3` was no named `build/config/pidgin3/pidgin3.ini`.

Reviewed at https://reviews.imfreedom.org/r/1824/