MenuItem: Fix disabled sub-menu item color

See #6610 comment:15 for details.

Firstly, don't save and restore the super menu's low color
since it never changes, instead, save and restore the super
menu's high color which does get altered here.

Secondly, set the high color to
B_MENU_SELECTED_ITEM_TEXT_COLOR only if the menu item
is activated and enabled, otherwise set the high color by tinting the
background color which might be
B_MENU_SELECTED_BACKGROUND_COLOR or
B_MENU_BACKGROUND_COLOR depending on whether
the item is selected or not.

Thirdly, complete the TODO by using a lighten tint if the menu
has a dark background and vice-versa.

This third item is actually a good candidate for a BControlLook
function because the disabled tint should depend on whether the
background color of the thing you're disabling is light or dark.