Qt: add a new class helper to open files
Qt: move playMRL slot from THEDP to RecentsMRL
Qt: use the new helper for Main Interface DnD actions
Qt: DialogsProvider, use the open helper
Qt: playlist, use the new helper to Open files
Qt: provide an helper to open items
Qt: use the open helper in the Open Dialog
Qt: use the open helper to open directories

And there was no need to use the Item version, and the MRL was bogusly
added to RecentsMRL
Qt: use the open helper for streaming
Qt: playlist enqueue, use the Open:: helper