gnome-xkb-info: Fix adding layouts to language and country tables

Layout->xkb_name isn't unique so we can't use it as a key in the hash
table. Layout->id is the unique identifier that we should be using,
otherwise some layouts would never get added.

https://bugzilla.gnome.org/show_bug.cgi?id=729210
gnome-xkb-info: Apply main layout locale metadata to variants

If a variant doesn't specify language/country metadata then we should
file it under its main layout's language/country.

https://bugzilla.gnome.org/show_bug.cgi?id=711291