PopupMenu width should not take into account Separator width in most cases

When using a control that has a PopupMenu and contains a Separator control, there is a situation where the width of the PopupMenu is far greater than what it should be. This occurs when the menu items pref width (ignoring the separator) is LESS THAN the separators pref width. In this situation, the width of the menu will actually be the pref width of the separator, when in reality we should be sizing the separator to fit the pref width of the actual menu items.

The only exception to this is of course when the menu ONLY has separator controls, which whilst admittedly is dumb, is a possibility. In this circumstance, we should size the width of the PopupMenu to be the width of the widest separator. Yes, I know this is dumb.