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 <jpnurmi@digia.com>