WIP issue #1173. Adding default suffix (extension). While at it, added selected filter support and removed unit test assert about its use.

QQuickWidget can’t handle this

Now works for more than Serial links

Moving methods from MainWindow to QGCApplication so that more things
can run without a window. Helpful for testing.

Emitting signal beforehand can lead to subtle bugs where code expects
value to be updated and it is not.

Starting point for real implementation