 ComboBox: support open popup centered or below button
 ComboBox: support open popup centered or below button

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