Patch for exporting an interface to alternative ticket workflows, and a plugin using it

All further development of the patch is now being undertaken on Trac's Trac under t:WorkFlow.

Description

This is a patch against Trac trunk which implements an extension mechanism for allowing alternative ticket work flows. It is discussed at length on PEC under #869 and NewWorkFlow.

The patch adds two things to Trac's Python API:

• The trac.ticket.api.ITicketWorkflow interface, which abstract the ticket action control logic into an extension point.
• The trac.ticket.field module which is used to describe ticket fields that can be passed to Trac's form_control() Clearsilver function.

It also adds one function to Trac's Clearsilver macros:

• form_control(control). This takes a trac.ticket.field.Field object and renders it as a form control.