Allow filtering of namespace by request on lexicon page

Like https://github.com/modxcms/revolution/commit/58ed1e52057ba71d020ffc4019778860958395e0 this enables users to filter on namespaces by request on the lexicon grid

Something like http://domain.com/manager/index.php?a=71&namespace=babel

This is useful since you can share a link to a specific lexicon e.g. from the dashboard, so clients can easily find it.