Move scene properties to property collection

[BradyAJohnston]

Moves all of the properties that were defined as bpy.context.scene.MN_import_* to instead by inside a MN property group, so they are now accessible via bpy.context.scene.mn.import_* which helps to not pollute the scene name space.

Moves the __init__.py contents into addon.py and only register() and unregister() are imported, to help clean name space.