applets/mediaframe: Port to Qt6
Another simple applet port:
- Port to Qt6 Dialog API, this was a little bit more involved but works fine. I tested it on both folders and files (although our folder selection is not ideal, you have to select folders from the outside).
- Remove QML import versions so Dialog gets imported correctly.
Note that configuration is not saved properly, but this is not an issue specific to the applet (it affects all of them currently)