[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: {F6387268, size=full}

Bottom panel, showing search results: {F6387270, size=full}

Top panel, open: {F6387274, size=full}

Top panel, showing search results: {F6387275, size=full}

Free-floating widget, open: {F6387276, size=full}

Free-floating widget, showing search results: {F6387277, size=full}

Reviewers: #plasma, broulik, #vdg, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, Codezela, plasma-devel

Tags: #plasma

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