[applets/kickoff] Improve appearance of draggable button grid in settings window

Summary:
The button grid was not centered, the content inside the buttons was not centered, the
vertical spacing was too high on top and too low on bottom, and the explanatory text
got cut off.

This patch solves all those problems.

Big thanks to @davidedmundson for his help getting this merge-ready.

BUG: 409957
FIXED-IN: 5.17.0

Test Plan:
Before: {F7247447, size=full}
After: {F7247446, size=full}

Reviewers: #vdg, #plasma, davidedmundson, GB_2, hein

Reviewed By: #vdg, #plasma, davidedmundson, GB_2

Subscribers: hein, GB_2, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23158