Put the label truncation code back in BMenuItem.

Just a few commits ago I moved the label truncation code out of
BMenuItem and into BMCMenuBar because the truncation had to happen
outside of BMenuItem. Turns out, that wasn't true so I'm moving the
label truncation back into BMenuItem and removing the _DrawItems()
method from BMCMenuBar.

Note that the code is not a copy of what was there before, but, the
updated version I created for BMCMenuBar. The main difference is that
I use menuPrivate.Padding() instead of GetItemMargins() and I always
use the width of the parent menu frame instead of using fBounds even
if the state is not MENU_STATE_CLOSED. These are changes needed for
BMCMenuBar but should work just as well for a regular BMenu.
BMCPrivate: Cleanup

Remove no longer needed header includes, most that I recently added
a few that were already there but just aren't needed anymore. Don't
use BPrivate::MenuPrivate namespace.