Fix assistant

Merged Daniel Garcia Moreno requested to merge (removed):fix-assistant into master

This MR fixes the assistant. It's broken because there's two pages with the same title. I've changed the title of the language configuration page and I've set the modal properties to show as a modal dialog.

This MR also includes a second commit that updates the GtrAssistant code to remove deprecated code, the bigger change is the use of G_ADD_PRIVATE.

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