[Kickoff] Don't respond to window resizing/tiling shortcuts

Summary:
This patch does for Kickoff what Kicker already does, and sets the maximum height
equal to the minimum height. This makes it no longer respond to window tiling and
maximize keyboard shortcuts.

BUG: 402614
FIXED-IN: 5.15.2

Test Plan:
Apply patch, restart plasmashell, open Kickoff, hit {key Meta PgUp}. Kickoff closes
and the shortcut is passed down to the active app instead.

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19023