moved provider selection popup menu to be at the top of the main window

for now, i just removed the label that was in its place. i never found the label status to be very useful anyway.
Then s/`QtWidget`/`QWidget`/ to be more generic (the type `QtWidget` does not exist).
It seems that the logic is tied to a combo box (variable names and methods used), so you should be explicit on the expected type until it changes. We will/should update the docstrings if the expected object type changes.

What other than a combo box could we use? Am I missing some case?