Note: I didn't fix all the connects and there are a few missing things.
This compiles, more work is coming.

Also move Designer::Internal::FormWindowEditor ->
Designer::FormWindowEditor.

Added IExternalEditor which knows a kind and a mimetype.
Make EditorManager and ProjectExplorer "Open With" query
the interface and add the respective kinds.
Add "openExternalEditor" to EditorManager.
Add External editors for Designer and Linguist,
making use of Mac 'open' or Designer's Tcp socket mechanism
to ensure files are opened in the same instance (per Qt version).

Task-number: 249392
Reviewed-by: con <qtc-committer@nokia.com>