Bug 19966: Add ability to pass objects directly to slips and notices

Bug 19966: Add ability to pass objects directly to slips and notices

Koha spends an incredible amount of time on parsing and processing parameters
passed in to slips and notices. It would be immensely more efficient to be able
to pass objects directly to GetPreparedLetter so it doesn't need to do any
fetching / processing on them.

Test plan:
1) Apply this patch
2) prove t/db_dependent/Letters/TemplateToolkit.t

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>