Restore correct menu border color

cac4b544 introduced drawing separators
of transparent QMenus with WindowText color with 0.25 opacity. While the change
in drawMenuItemControl uses the correct condition, drawPanelMenuPrimitive
checked whether the widget had an alpha channel. This is wrong because we set
Qt::WA_TranslucentBackground always on all menus. The very low opacity made it
look like the section headers, separators and highlight were drawing outside of
the menu.
BUG:428710