This method is used to get the sheet's name and to display it
in the tabs (sheet selector)
I've also added an unit test for it and uncommented a similar one

Change-Id: Ia866815c594a873812c71a6c86e303c869e1f093
Reviewed-on: https://gerrit.libreoffice.org/17294Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan Holesovsky <kendy@collabora.com>