Refactor Window vs tkinter dependency

[tmontes]

Facts

• The work in Turtle forward movements default to tracking angle changes. #79 / Canvas Y increases towards the top of the screen #80 is decoupling the Window class from its direct tkinter.Canvas dependency -- keeping a tkinter.Canvas-like interface, but decoupled.
• The Window class is still directly coupled with tk.Tk and tk.TopLevel for OS level window creation.

Idea

• Decoupling the second point, might facilitate future implementations of non-Tk GUI backends, like Qt, and more (web browser? maybe problematic because current GUI API is synchronous)