Do not show the crumbs again after hiding them

After the changes in the previous commit "_showCrumb" no longer shows
the menu, only the same crumb that was hidden by the last call to
"_hideCrumb". Therefore, if the crumb was hidden because it did not fit
there is no need to try to show it again, as it will still not fit.

Moreover, the calculated width for a hidden element is not always
accurate; in some cases the calculated width is lower than the actual
width (it happens, for example, when using a background image like the
"Share" icon), which causea the crumb to be shown even if there is not
enough room, which in the end causes the siblings to overflow the
contents.

No unit tests for this one, though; you will have to trust me on this,
sorry ;-)

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