VS: Restore header files marked as OS X Framework content (#13196)

Header files listed in a target's PUBLIC_HEADER or similar properties
are marked as OS X Framework content.  Refactoring performed by

 commit 11d9b211 (Add cmGeneratorTarget to represent a target during generation, 2012-03-07)
 commit 45c2f932 (Simplify cmMakefileTargetGenerator using cmGeneratorTarget, 2012-03-07)
 commit 328c0f65 (Simplify cmVisualStudio10TargetGenerator source classification, 2012-03-19)

and related commits accidentally removed such files from treatment as
normal header files by the VS generator (generators other than Makefiles
and Xcode).  Move handling of such files out of cmGeneratorTarget and
back to cmMakefileTargetGenerator.  The central cmGeneratorTarget
classification will always treat them as header or extra sources.

Source/cmGeneratorTarget.h

@@ -37,7 +37,6 @@ public:
   std::vector<cmSourceFile*> HeaderSources;
   std::vector<cmSourceFile*> ObjectSources;
   std::vector<cmSourceFile*> ExternalObjects;
-  std::vector<cmSourceFile*> OSXContent;
   std::vector<cmSourceFile*> IDLSources;
   std::string ModuleDefinitionFile;