ZF-14: Remove Zend_Pdf_Const
Zend_Pdf_Const was initially conceived as the single home for all PDF-related constants. This worked well enough when the number of constants was small, but recent work (ZF-6 and Zend_Pdf_Image subclasses) has more than tripled the constant count, making a master class like this difficult to maintain.
None of the other components in the framework share this model; they all define constants in the base classes that use them. For consistency with the other components in the framework, and to improve maintainability going forward, move the constants currently defined in Zend_Pdf_Const to the base classes in which they are used.