WebPositive: Don't display '...' icon when empty

* Conditionally add/remove fOverflowMenu to BookmarkBar when window
is resized or a bookmark is added/removed.
* Improve handling of large window size changes (e.g. zoom button)
which require moving multiple bookmarks to/from fOverflowMenu in a
single call to FrameResized.
* Fixes #17616.

Change-Id: Id3e991167ec8d7aa540dd0644749cad1612de293
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5324
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

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.

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.

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.