The sidebar has width limit, so a panel shouldn't attempt to
resize more than that. Otherwise we'll get an endless loop.

(based on e98cf0c6)

Change-Id: Ia36535637e3585595c673c7fc46a1a7b162b74ba
Reviewed-on: https://gerrit.libreoffice.org/17667Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>