moved some functionality to GLFW.jl

[louisponet]

Start of restructuring.

• move gl_createcontext out of GLWindow.jl/screen.jl to a constructor for GLFW.Window.
• moved a lot of GLFW.Window related functionality from GLWindow.jl/screen.jl,types.jl,core.jl to GLFW/types.jl.
• Moved MonitorProperties from GLWindow.jl/types.jl to GLFW/types.jl
• Changed MonitorProperties so it doesn't depend on GeometryTypes: Vec hit in performance shouldn't matter imo.
• moved general GLFW.jl related functionality from GLWindow/screen.jl,core.jl to GLFW/extensions.jl

Tests work, also ran the tests for the pullrequest in GLFW.jl

General Idea: Firstly, splitting pure GLFW.jl related functionality from GLWindow.jl will allow it to be less backend specific, and act as a interface to different OpenGL window/context providers. This is in light of hopefully seperating Windowing functionality in 'GLWindow' from GLAbstraction.jl and provide more of an API experience.
Secondly, I understand this makes the package no-longer a barebones wrap around the GLFW library, but was it really purely that to begin with, and would people not use the added functionality that only depends on GLFW if it is there?

If the authors of GLFW.jl want to keep it a barebones library, as is right now, we might be mimicking these changes into a folder backends where the backend specific things are implemented, or submodules...