[Plasmoid] Clean up layout

Summary:
This patch cleans up the Search plasmoid's layout to essentially match that of KRunner (but without the side buttons).

It also cleans up the layout-related code a bit as there were some workarounds and hacks that are no longer needed.

Test Plan:
Bottom panel, open:

Bottom panel, showing search results:

Top panel, open:

Top panel, showing search results:

Free-floating widget, open:

Free-floating widget, showing search results:

Reviewers: Plasma, broulik, VDG, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, Codezela, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D16622