dnn: added "hidden" experimental namespace

Main purpose of this namespace is to avoid using of incompatible
binaries that will cause applications crashes.

This additional namespace will not impact "Source code API".
This change allows to maintain ABI checks (with easy filtering out).

modules/dnn/src/init.cpp

@@ -42,10 +42,9 @@
 #include "precomp.hpp"
 #include <opencv2/dnn/layer.details.hpp>
 
-namespace cv
-{
-namespace dnn
-{
+namespace cv {
+namespace dnn {
+CV__DNN_EXPERIMENTAL_NS_BEGIN
 
 static Mutex* __initialization_mutex = NULL;
 Mutex& getInitializationMutex()
@@ -98,4 +97,5 @@ void initializeLayerFactory()
     CV_DNN_REGISTER_LAYER_CLASS(Scale,          ScaleLayer);
 }
 
-}} //namespace
+CV__DNN_EXPERIMENTAL_NS_END
+}} // namespace