Fix batch search not showing products without manufacturer

ProductManufacturer needs to be a LeftJoin or else it will filter batches
without manufacturers.

Change-Id: I173bb4dae90fa1f2b0070c7f8787ff42cfa9ffdd
Add a better api for print reports on searchs

Instead of checking for 'on_print_button_clicked', define a report_class since
the majority of searchs duplicate the same code.

Also, this makes it possible to avoid having a print button on a subclass of a
search that has one (next commit will have a case like that).

Change-Id: Ia283704f1e9928b24e3e262ff9fc8341a5dd223a