Refactor code to handle light mode as a separate class

[goanpeca]

Currently the code inside spyder.py is a bit convoluted because if not light appears in a lot of places.

This change would make the code easier to read and maintain, and still keep the light mode in case there is people still using it.

[ccordoba12]

We should put a date to this one. I'd prefer to have it for 3.0, to test things very well and don't add possibly disruptive changes in 3.1.

Thoughts @goanpeca?

[goanpeca]

Should be quite fast to do, so I will do it tonight and we can move on with fast things first.

We still have a pending hangout to go over projects, let me know when ;-)

[ccordoba12]

We still have a pending hangout to go over projects, let me know when ;-)

Yep, I know, but please focus first on issue #2555, so we can release beta1.

[ccordoba12]

I finally decided to remove light mode. See PR #2874 for my reasons.