Use Kirigami Cards for grid delegates, with visible buttons

KCM grid delegates right now have buttons that appear on hover. Hover buttons have questionable usability, since they are not super discoverable and not usable at all on touch, where we currently just make the buttons unconditionally visible, which looks a bit ugly and heaby with the current design.

For these reasons, we have been moving away from hover buttons for some time, making list view buttons non-hover in conjunction with a minor redesign few Frameworks releases ago. So it's time to do the same thing for grid delegates.

Multiple rounds of VDG attempts to make the buttons always visible with the current design or a tweaked version of it failed because it clashed with the existing style. So VDG produced a new mockup of a more card-style delegate, which has been implemented here using a Kirigami AbstractCard for consistency.

Looks good with all existing grid delegates, even the ones with non-standard sizes. Examples:

Closes plasma/plasma-workspace#2

Co-authored-by: Nate Graham nate@kde.org

cc @teams/vdg