Single column mode, 2 columns by api

most KCMS don't work right in 2 columns, and all old ones for sure shouldn't be. some like Users instead make use of 2 columns and rely on it. this makes kcms to use single column mode by default, then if a kcm has in its main element: Kirigami.ColumnView.fillWidth: false it will go to 2 columns mode

see also kdeclarative!46 (merged) and plasma/plasma-desktop!308 (merged)