[svn-r3122] Purpose:

Fix and improve

Description:
        - Put functions that are common to H5File and Group into a
          prototype class, CommonFG.  I didn't do that before because
          of the fear of the consequences of multiple inheritance, since
          H5File and Group already inherit from different super classes.
          I recently read a C++ book and learned to use MI more safely.
          This change reduced some more of code redundancy.
        - Added missing const to some function parameters

Platforms tested:
        Solaris/CC 5.0 (arabica)

c++/src/H5IdComponent.h

@@ -12,7 +12,7 @@ class IdComponent {
    public:
 	// Parent classes must reset the current IdComponent copy
 	// before setting new id to control reference count
-	void setId( const hid_t new_id );
+	void setId( hid_t new_id );
 
 	// Pure virtual function so appropriate close function can
 	// be called by subclasses' for the corresponding object