This adds the ability to switch between different application-configurations
without interfering with Blender's normal operation.
This commit doesn't include any templates,
so its mostly to allow collaboration for the Blender 101 project
and other custom configurations.
Application templates can be installed & selected from the file menu.
Other details:
- The `bl_app_template_utils` module handles template activation
(similar to `addon_utils`).
- The `bl_app_override` module is a general module
to assist scripts overriding parts of Blender in reversible way.
See docs:
https://docs.blender.org/manual/en/dev/advanced/app_templates.html
See patch: D2565
