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:

After:

Reviewers: apol, VDG

Reviewed By: apol

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