[FifteenPuzzle] Port configuration window to QQC2 and Kirigami.FormLayout and improve UI

Summary:
- Port to QQC2 and Kirigami+FormLayout
- Use radio buttons to choose color vs image
- Disable inactive controls appropriately

Test Plan:
{F6791372}

Everything still works.

Reviewers: #vdg, #plasma, GB_2, filipf

Reviewed By: #vdg, GB_2, filipf

Subscribers: GB_2, filipf, plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D20747