Don't use "import" in string representation of objects

We should not have "import" in this line:
https://github.com/enthought/mayavi/blob/61ef7c0eebbdca474b7cfdf6082bfc17fb5aaf59/tvtk/plugins/scene/ui/actions.py#L20-L23

instead we can simply have `'tvtk.plugins.scene.i_scene_manager.ISceneManager'`. I am unsure if this is a one-off occurrence or if there are many other instances of this across the code base.