Make Primary Combo a regular QComboBox
Summary:
This will make it easier to re-design the UI.
Also it centralizes all updates, before a bunch of connections for the
individual outputs were not made when a new output appeared.
Reviewers: Plasma, mart
Reviewed By: Plasma, mart
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D14023