kcmcontrols: add "framelessInnerView:" property
This property facilitates "frameless" KCMs that want to draw their inner scrollview or background with no frame and touch the edges of the surrounding UI components.
It is false by default, meaning there are no visual changes to anything at the moment. However KCMs can opt into it when they are ready to remove the frames on their inner scrollviews, which will improve their appearance by removing an unnecessary frame and make them look more like the frameless pages of modern Kirigami apps.
Example of a KInfoCenter KCM using this property:
Example of a System Settings KCM using this property:
