Allow dcx::window struct to be subclassed step 1. This step moves the definition for the window struct into window.h, keeping all member variables private and declaring the window related functions as 'friends'. This avoids changes to source files that use the window struct at this stage. Also moving some of the simpler functions in window.cpp into the struct definition to inline them. I would prefer to use class methods, so changing some of these to class methods as well.
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