[Plasmoid] Clean up layout
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.
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:
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, Codezela, plasma-devel
Differential Revision: https://phabricator.kde.org/D16622