make the logic of PageRow a ListView

Summary:
it implements internally things like snapping and layouting
makes the qml code significantly simpler
it needs the columns stay the same size (as listview doesn't
correctly supports items of eterogeneous sizes)

Test Plan: tried the widget gallery on different styles, desktop and mobile

Reviewers: Kirigami

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D3561