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