[interfacekit] Missing stuff from GraphicsDefs.h

Hi there,

it seems that all extern constants in GraphicsDefs.h have not been 
defined anywhere yet.
Should they be defined in the local files, i.e. B_MAIN_SCREEN_ID in 
Screen.cpp, or together in a GraphicsDefs.cpp?
If no one stops me, I will commit the latter to the repository in the 
next few days.

Bye,
   Axel.

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