Move Widget search field to its own row so it doesn't get compressed

Summary:
Move search to its own row to prevent it from being compressed

BUG: 393427

Test Plan:
- Recompile `plasma-desktop`
- Bring up Widget Explorer
- Press {nav Search} to see Search in new row
- Select widget type category to see Title become elided

Reviewers: ngraham, davidedmundson, #plasma

Reviewed By: ngraham, davidedmundson, #plasma

Subscribers: abetts, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12855