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.

Reviewers: mgallien, Elisa

Reviewed By: mgallien, Elisa

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