Web+: move BookmarbKar to own file.

It's a bit long for an inline class, and I'm going to add more stuff to
it.
Move overflowing items from bookmark bar to a menu.

Since the bookmark bar can now be resized smaller than the space needed
for the items, move items that don't fit in it to a menu, which is
always the last item in the BMenuBar.
Convert strdup.c to C++.