applets/kickoff: make top corners and left/right header paddings perfect
Right now these are imperfect due to fancy math on PlasmoidHeading that it isn't compatible with. Switch to a simpler implementation that allows us to perfect the top corners and the left and right paddings of the toolbar items.
BUG: 473061 FIXED-IN: 6.3.0
(cherry picked from commit 8df77018)
Co-authored-by: Nate Graham nate@kde.org