connectionitem: delay tabbar height binding till after completion
slightly complicated interaction of bugs here. something inside tabbar, listview, or listview highlight is buggy such that the highlight breaks when the initial height is 0.
to work around the problem we set the initial height to the implicitHeight (i.e. the contentHeight) and only install a binding that may be 0 when the component is complete. effectively delaying the zeroness.
this should ensure the tab highlight appears in all Qt versions