Backout changeset 14e7e8f7a2e5 (see note)

Apparently PythonService.exe doesn't acknowledge the 'absolute_import' feature
from __future__...? So we can't have a 'rattail.sqlalchemy' module and instead
must keep 'GPCType' within 'rattail.gpc'.

extensive commit (see note)

The following changes are included:

- Initial batch support added (with ``PrintLabels`` provider).

- GPC data type added.

- Changed internal name of file monitor.

- Added progress support for label printing.

- Label profiles moved from config to database model.

- Removed ``rattail.db.init_database()`` function.

- Moved some enum values from db extension to core (``rattail.enum`` module).

- Improved SIL support: moved ``rattail.sil`` to subpackage, added ``Writer
class etc.