[kcms/feedback] Improve default kcmshell window size

Summary:
BUG: 417747
FIXED-IN: 5.18.1

Test Plan:
Better default size when showing the most info:

However window is too now big when showing the least amount of info:

...But that seems kind of unavoidable unless we want to dynamically resize the window,
which people seem to hate.

Reviewers: Plasma, VDG, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27448