Move QQuickControl::focusReason into QQuickItem

The "focus" concept is part of the Qt Quick framework, but QQuickItem provides no means to inform QML code about the Qt::FocusReason in the focusChanged or activeFocusChanged signals. Adding a parameter to those signals can only be done for Qt 7, as it breaks compatibility to do so earlier (QQuickItem is a public C++ class).

QQuickControl has a focusReason property that should live directly in QQuickItem, at the cost of storing an additional enum value for all items. That focusReason as a stored property only exists in QQuickControl means that QtQuick's event delivery machinery cannot ensure that the focusReason of the control is in sync with the events delivered, which results in issues like QTBUG-75862.