Improvement/task spec attributes

[essweine]

This PR includes substantial changes to the package structure and updated bpmn task attributes.

Package structure changes:

• all of the old bpmn task spec classes have been moved inside mixins
• inheritance from BpmnSpecMixin has been removed in the task-specific classes
• default classes that inherit from the mixin classes are in SpiffWorkflow.bpmn.specs.defaults
• specs that control the workflow but do not appear on the diagram are in SpiffWorkflow.bpmn.specs.control

Removing the BpmnSpecMixin inheritance makes it easier to extend bpmn task behavior without re-implementing task specific behavior.

BPMN Task attribute changes:

• all bpmn task specs (visible specs and controlling specs that don't appear on the diagram) have the same set of attributes
• bpmn_id and bpmn_name attributes are set when they are present in the diagram
• is_engine_step method has been removed in favor of the core task spec attribute manual
• spec_type method has been removed and the spec description it used to return is now in the description attribute, and configurable via the parser.
• id attribute has been removed from all task specs

The name attribute is still used internally by Spiff as a unique identifier for a task spec; id was redundant and confusing, so I've removed it (this enforces the distinction between tasks and task specs too --specs have names, tasks have ids). If a spec has a visible diagram element, it will have bpmn_id set; otherwise it will be None.

It makes a lot more sense to set the display name of the spec type at parse time (do you really want to create a subclass just to change this?); these are now determined by the XML element type and configurable in the parser.

The attribute changes should be updated automatically if a workflow with the older format, but to apply changes en masse, the convert_simple_tasks and update_bpmn_attributes functions in SpiffWorkflow.bpmn.serializer.migration.version_1_2.py can be used.