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