Locales class
New set of rules for projects and sets

In this new proposal we're trying to preserve the `gp_action` query_var
with the method names from GP_Route_{entity}::{method} in GlotPress'
`router.php`, and let it control the query part.  Post actions have
always a `/post/` in the end.

From this, we can also add subprojects support building a 'project dir
parser' for `$project_match`. Although I don't think it's elegant to keep
this parser logic here.

See ticket #3