Revamp grid browser delegates to improve UI

Summary:
This patch changes the grid browser delegates in the following ways:
- Labels are centered
- Album art size increases from 100px to 170px, making the album view more attractive and leaving more room for labels
- Hover buttons now appear in the corner, have borders, and no longer make the album art get washed out when the delegate is hovered
- Some minor layout and positioning bugs are fixed

As a result, the following bugs are fixed:
BUG: 406468
BUG: 406476

Also, the last usage of QQC1 in this component is removed as a side effect.

Test Plan:
Everything still works and there are no behavioral changes.

{F6780036}

Reviewers: mgallien, #elisa

Reviewed By: mgallien, #elisa

Differential Revision: https://phabricator.kde.org/D20581