Fix window height of Screen Locking KCM

Summary:
An implicitHeight must be set on QML components to prevent window size being too small.

BUG: 400355
FIXED-IN: 5.16.0

Test Plan:
  # Open Screen Locking settings
  # Go to Appearance tab
  # Two rows of thumbnails with labels should be shown

{F6727233}

Reviewers: davidedmundson, ngraham

Reviewed By: davidedmundson, ngraham

Subscribers: ngraham, abetts, filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20086