Make sure Panel cannot be smaller than the margins of the panel SVG

Depends on:
- plasma/plasma-workspace!2329
- https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/647

This commit reads the minimum size the panel can be drawn at in the Panel view
(so that it can be read by the panel cpp code) and makes sure that the thickness
box cannot set a value that's lower than that. Even if you change to a theme that
has min size bigger than current panel thickness, the value will update so that
it's never lower than it (since we re-check the thickness every time the min size
changes; see plasma/plasma-workspace!2329 (diffs)).

BUG: 461506
FIXED-IN: 5.27