Now it is possible to select a `custom` validation in the validation
dialog: this type of validation let's the user to define a formula,
the cell content is valid when the formula is evaluted to true, and
not valid when evaluated to false.

The `cutom` validation is correctly saved and restored for ods
documents, and is correctly imported and exported to xlsx documents

This patch contains an adaptation of a preliminary work of Justin Luth
for importing custom validation from xlsx documents. Thanks Justin!

Change-Id: Idc26654ba69a6f73d1b208d63acdad4b880c776d
Reviewed-on: https://gerrit.libreoffice.org/49084Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>