[Cuttlefish] Port toolbar to QQC2 which fixes combobox text being corrupted or invisible

Summary:
BUG: 366543
FIXED-IN: 5.16.0

Test Plan:

Reviewers: sitter, hein, Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20188