Show a vertical menu for the panel widget options pop-up

Summary:
This patch replaces the UI for the panel widget options pop-up with a vertical menu, replating the current horizontal bar with icons on either side of a label. This yields the following improvements:
- Bigger hit areas; easier to click and much more touch-friendly
- Buttons have text labels; easier to tell what they do
- Fewer changes of direction required when mousing; it's possible to access all actions with only vertical motions

Test Plan:
All buttons still work.

Before:
{F6108835}

After:
{F6112474}

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: gregormi, abetts, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14145