Refactor: forward-declare element type in collection headers

Move the element-type include from each migrated collection header into
its .cpp, leaving only a forward declaration in the header. Follows the
pattern from 9e4d79159 for RimJobCollection, made possible by the
constructor-time PdmObject base check.

ApplicationLibCode/ProjectDataModel/EnsembleFileSet/RimEnsembleFileSetCollection.cpp

@@ -18,6 +18,7 @@
 
 #include "RimEnsembleFileSetCollection.h"
 
+#include "RimEnsembleFileSet.h"
 #include "RimTools.h"
 
 #include "cafCmdFeatureMenuBuilder.h"