Reimplement Languages page in QML

Installation of missing languages is not implemented here, but will be
implemented either in the "final" page, just before we apply all changes
and finish the wizard, or in the background after user leaves the page.

The code for that is available in master, but will require 1) Qt 5 version
of PackageKit-Qt5 and 2) some simplifed wrapper around it for use from QML.