[Trash KCM] Clean up and standardize UI to be in line with the KDE HIG

Summary: Clean up and standardize the UI to be in line with the KDE HIG.

Test Plan:

• All functionality still works
• Checking and unchecking the checkboxes still enables and disables companion items appropriately
• No unit test regressions, and trash test still passes
• Visual comparison:

Before:

After:

Reviewers: Dolphin, Frameworks, VDG, davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, abetts, rkflx, kfm-devel, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12986