Detect previewing cell from different class

[Splines]

The word "currently" refers to code of #8 in this pull request.

Currently, when previewing a cell from one class, then previewing a cell from another class, the new elements are just put on top of the active scene. Instead, we'd probably want to ask the user if they want to quit the active Manim interactive shell and run the manimgl command again with the new class name.

We could reuse code that would be introduced to close #2 beforehand.

[bhoov]

I would also treat this as an enhancement that is not necessary for v0.1.0 -- like Jupyter notebooks, I think it is reasonable for users to understand that they are editing the global state of a backend kernel in the order that cells are run

[Splines]

With the reload() command in #77, this would be very easy to implement. See also my PR here in the Manim repo. Sorry, nope wouldn't work as manimgl takes the Scene as command parameter, so we cannot just reload(). So we'd have to start entirely. But I do agree that this is really not urgent.