PC3: Clean up bindings on padding
PC3/ComboBox: Drop pre Qt 5.9 workarounds
The ComboBox::editable property was introduced in QtQuick.Controls 2.2 (Qt 5.9).
PC3: Mark some properties as required
The ListItemBackground component is not functional without an associated control; just like ButtonContent does not make sense without a button.
PC3: Better, safer bindings for padding
Item::background is an Item property; it may or may not be an instance of KSvg.FrameSvgItem, depending on how clients override it. Also use undefined as a fallback so that it invokes RESET for maximum flexibility.
Links to: !1117 (merged)