ComboBox: Get rid of 'effectivePressed' binding loop
ComboBox: Get rid of 'effectivePressed' binding loop

The binding loop looked like this,

    ButtonBehavior.effectivePressed = true
    BasicButton.pressed = true
    call Menu.showPopup()
    ButtonBehavior.containsMouse = false (mouse grabbed by popup)
    ButtonBehavior.effectivePressed = false

We remove any dependency to ButtonBehaviour and BasicButton, thus
also cleaning the ComboBox API to the bare minimum.

Change-Id: I1cfedcfe63b2835a982a66928b840fe726e89931
Reviewed-by: J-P Nurmi <b84a892f57a298bb5dae653b4e188948a84d5549@digia.com>

richardmg/qtcomponents,kobolabs/qtquickcontrols,mizukusak/qtquickcontrols,richardmg/qtcomponents,jfaust/qtquickcontrols,drgvond/qtquickcontrols,richardmg/qtcomponents,mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,drgvond/qtquickcontrols,drgvond/qtquickcontrols,kobolabs/qtquickcontrols,kobolabs/qtquickcontrols,jfaust/qtquickcontrols,jfaust/qtquickcontrols