[-] FO : search combination upc, ean13 too

Already searches combination reference and product upc and ean13 but not combination upc and ean13. Also added supplier_reference to be thorough and consistent with back office search and product searchByName
[-] BO : search combination upc and supplier_reference

For consistency, combination reference and ean13 is already searched, but not upc and supplier_reference, which are searched in the product record above.
[-] WS : allow adding product to empty category

When Web Service adds a new product via POST, this function gets called with $position=0, but when category is empty count($result)=0, the previous test always fails and the product cannot be added. Doesn't seem to happen when product is added via BO interface to an empty category.
Merge pull request #2034 from tmackay/my-changes

[*] BO : Search field consistency with combinations and fix for WS