Use "frameless" style for grid and scroll view QtQuick KCMs by default

This commits makes GridView and ScrollView KCMs look like modern
frameless Kirigami-style views by removing the extra frame around the
scrollview. This styling change is opt-out; grid and list view KCMs are
now frameless by default.
2 jobs for !91 with frameless-scrolly-kcms in 1 minute and 45 seconds (queued for 13 seconds)
latest merge request