Initial support for Python/Manipulator integration
from 'custom-manipulators' branch.
Supports:
- Registering custom manipulators & manipulator-groups.
- Modifying RNA properties, custom values via get/set callbacks,
or invoking an operator.
- Drawing shape presets for Python defined manipulators (arrow, circle, face-maps)
Limitations:
- Only float properties supported.
- Drawing only supported via shape presets.
(we'll likely want a way to define custom geometry or draw directly).
- When to refresh, recalculate manipulators will likely need
integration with notifier system.
Development will be continued in the 2.8 branch
