Bug 11459: use drop-down to set OrderPdfFormat syspref

This patch converts the OrderPdfFormat to a choice.

To test:

[1] Look at OrderPdfFormat in the system preferences editor. Verify
    that there is a drop-down giving a choice among the three PDF
    basketgroup printing formats.
[2] Change the OrderPdfFormat setting and print a basketgroup.  Verify
    that the chosen template is used.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>

I followed the test plan. Patch behaves as expected.
Signed-off-by: Marc Véron <veron@veron.ch>

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Improves usability of the OrderPdfFormat system preference.
Works as described, only changes YAML file.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 3b7349e59005696094d5f7019583211140a355f0)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>