Migrate wallpaper config UI to Qt Quick Controls 2 and Kirigami

Summary:
KCM UI is gradually moving to Qt Quick Controls 2 components and Kirigami form layout. This can solve some layout consistency issue and HiDPI rendering issue (QQC1 doesn't render properly in HiDPI screen).

{F7779229}

Reviewers: #marble, #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, marble-devel, kde-edu

Tags: #marble, #kde_edu

Differential Revision: https://phabricator.kde.org/D25474