Fixed position of insert new sheet button (+) when using a sheet with
a different directionality than that of the interface.

Change-Id: Ifc7020c4eb7a1686c23b4d827c47494b88a236b5
Reviewed-on: https://gerrit.libreoffice.org/1385Tested-by: Lior Kaplan <kaplanlior@gmail.com>
Reviewed-by: Eike Rathke <erack@redhat.com>