UI layout fixes to mer option menus

Qt macstyle had some different default values, when compared to styles used on other platforms.
If the fieldGrowthPolicy is set to QFormLayout::ExpandingFieldsGrow, then it seems that the hsizetype must be "Expanding" for the QLabel to grow. By default only QMacStyle uses QFormLayout::FieldsStayAtSizeHint, which sets the maximum size of the fields to sizeHint. So there could be an issue with the sizeHint value of the QLabel widget. This doesn't fix it, but at least it should make the behavior consistent on all 3 platforms.