Use QDialog as base for AKM.

This allows the akm to be treated as a preferences dialog instead of an
extra 'main window'.
Show specific note for providers not supporting mx.

Also add a helper method to disable the ui and show a message.
Add changes file for #5513.
Merge remote-tracking branch 'refs/remotes/ivan/feature/5513_update-note-in-akm' into develop