Fix navigation regressions

Summary:
This patch fixes navigation regressions caused by larger cards in the gallery.

• Keyboard focus is properly handled
• Cards collapse to a smaller form when a second page is present
• A home button is present to allow returning to the large-form cards

Test Plan:

Reviewers: VDG, Kirigami, ndavis, mart

Reviewed By: VDG, Kirigami, ndavis, mart

Subscribers: mart, ndavis, ngraham, plasma-devel

Tags: Plasma, Kirigami

Differential Revision: https://phabricator.kde.org/D28888