Bug 6936: Allow multiple itemtypes for bib record export

Bug 6936: Allow multiple itemtypes for bib record export

This patch allows multiple item types to be chosen when exporting
bib records from the Tools module.

Test plan:
0. Apply patch and koha-plack --restart kohadev
1. Go to http://localhost:8081/cgi-bin/koha/tools/export.pl
2. Select "All" for "Item type" and note 435 bibs are exported
3. Select "Books" and note 360 bibs are exported
4. Select "Books" and "Visual Materials" and note 380 bibs are exported

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>