[kcm/keys] Use AbstractKCM as a base, rather than ScrollViewKCM

AbstractKCM was made for exactly this use case (one big view with
internally-managed scrollviews). We should use it.

This also corrects the margins.

kcms/keys/package/contents/ui/main.qml

@@ -24,10 +24,10 @@ import QtQml 2.14
 import QtQml.Models 2.3
 
 import org.kde.kirigami 2.12 as Kirigami
-import org.kde.kcm 1.2 as KCM 
+import org.kde.kcm 1.4 as KCM
 import org.kde.private.kcms.keys 2.0 as Private
 
-KCM.SimpleKCM {
+KCM.AbstractKCM {
     id: root
     implicitWidth: 800
     implicitHeight: 600