port to PlasmaComponents3

Summary:
the pieces of old components we couldn't control, like the combobox menu
didn't react to palette change

BUG:397845

Test Plan: applet still works and looks identical, combo popup changes color dinamically

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15165