Fix menu visibility

The crumb for the menu was shown like any other crumb when calling
"_showCrumb", but it was also shown when other crumbs were hidden
without taking into account the available width. This caused several
related problems, like the breadcrumbs taking too much space when the
menu was sometimes shown after the rest of the crumbs were adjusted to
the available width, or the menu being shown instead of the last crumb
even if there was room for it when the available width was increased.

Now the menu is always hidden before starting the resizing of the crumbs
to ensure that whether it was previously shown or not does not affect
the result. In a similar way, the menu will no longer be shown by
"_showCrumb", as it is not a regular crumb that has to be shown simply
if there is enough room. The menu is now shown as soon as any other
crumb is hidden; this ensures that the menu width will be taken into
account in further width checks. As when _updateMenu" is called it no
longer needs to take care of showing the menu this fixes the issue
revealed when fixing the test setup in the previous commit.

Finally, this implicitly fixes the failure in the breadcrumbs tests when
run on Firefox, as it was caused by the menu interfering in the
calculations of the other crumbs when increasing the width.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>