Fix incorrect Kickoff tab bar layout for vertical panels

Summary:
This fixes the broken layout of the tab bar (tab bar takes the whole view)
when a panel is changed from horizontal to vertical that persists
until plasmashell is restarted.
BUG: 393888

Test Plan:
Change panel orientation from horizontal to vertical. Open Kickoff.
Before fix: tab bar fills the whole view making the Kickoff unusable even if you make the panel horizontal again.
After fix: tab bar has the correct size.

Reviewers: #plasma, hein, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T10047

Differential Revision: https://phabricator.kde.org/D22988