Overhaul left drawer's UI

Summary:
This drawer needed less work than the right drawer but could still use some
improvements, which include:
- Improve the spacing of the effects grid items
- Increase each item's size so you can actually see what it does
- Add a border around each effect for aesthetics
- Improve header margins

Test Plan:
Before: {F6540141}

After: {F6540144}

Reviewers: apol, #vdg

Reviewed By: apol

Differential Revision: https://phabricator.kde.org/D18190