...instead of in BMenuItem and remove the truncation code from BMenuItem.
The label truncation code cannot work in BMenuItem because the super
menu helpfully resizes itself to fit the menu item. So, instead we do the label
truncation in BMCPrivate making sure that BMenuItem there can't expand the
BMCMenuBar because we set the width to fMenuField->_MenuBarWidth()
explicity.
Note that this only truncates the label in BMCMenuField, i.e. the label inside
the menufield, it does nothing to the labels of the menu items in the attached
BMenu or BPopUpMenu which is exactly what we want.
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