Move FWEveDigitSetScalableMarker delcaration in header file. Related to #332

In pull request #332 were problems with dictionary since Fireworks/Core/src/FWEveDigitSetScalableMarker.cc file was included in classes.h. 

I have split the code into source and header so that classes.h include only header files. As I understand a workaround for problem in #332 has already been made, but not included in the official cmssw repository.