Don't let the Advanced tab's spacers expand all they want

Summary:
We have 3 vertical spacers in the Advanced tab and they're defined as Expanding.

So that means they become quite the gaps if the window is expanded.

This patch changes their size policy to Preferred to avoid such a feature.

BUG: 417657
FIXED-IN: 5.18.1

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27418