Manipulator: Python API

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

doc/python_api/sphinx_doc_gen.py

@@ -332,6 +332,9 @@ except ImportError:
 # to avoid having to match Blender's source tree.
 EXTRA_SOURCE_FILES = (
     "../../../release/scripts/templates_py/bmesh_simple.py",
+    "../../../release/scripts/templates_py/manipulator_operator.py",
+    "../../../release/scripts/templates_py/manipulator_operator_target.py",
+    "../../../release/scripts/templates_py/manipulator_simple.py",
     "../../../release/scripts/templates_py/operator_simple.py",
     "../../../release/scripts/templates_py/ui_panel_simple.py",
     "../../../release/scripts/templates_py/ui_previews_custom_icon.py",