Rework instantiation of MotionBuilderConfigOverlay

[rocco8773]

Rework the instantiation of MotionBuilderConfigOverlay such that a valid MotionBuilder object is instantiated and linked to the display by the end of __init__.

The generated MotionBuilder object is either a copy of the motion build attached to the given MotionGroup or (if MotionGroup has no motion builder) a default motion builder defined by the _default_motion_builder() method.

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• Reorganize the widget _init_* methods to be list alphabetically.
• Created _init_motion_builder to do the full MotionBuilder instantiation.
• Created _init_mspace_display to do the MotionSpaceDisplay instantiation.
• Created link_motion_builder_to_display to do the associated linking.
• Morphed _initialized_motion_builder into _default_motion_builder. Now, _init_motion_builder does the full instantiation and _default_motion_builder just gives the default motion builder (as needed).
• Added kwarg skip_assignment to _spawn_motion_builder so "spawn" can be used by _default_motion_builder and (thus) _init_motion_builder.