Clean up Window interface

[hhirsch]

The Window class is messy to use.

• When you use it without running setConfig you'll do method calls on nullptrs leading to segfaults
• There is input code crammed in there

We want the following changes:

• Erase dependency to the Config class altogether using reasonable defaults, setters and an intermediate (named WindowConfigurator) class that applies config values to setters of the Window class.
• Move the input handling stuff to a different class
• Dramatically decrease LOC in Window class
• Move main functions to source/core/gl
• Possibly move into own repository