Menu: Fix zero-sized popup glitch in RTL environment

For some reason ListView fails to report any sane contentHeight
specifically in RTL mode.

There is a similar trick to force it to work in recently added
HorizontalHeaderView.qml styled override. But unlike what that comment
in header view says, Menu actually creates components (you could
inspect them in GammaRay), but somehow they are being ignored for the
purposes of contentHeight estimation.