Merge pull request #13332 from mshabunin:dnn-backends

DNN backends registry (#13332)

* Added dnn backends registry

* dnn: process DLIE/FPGA target

modules/dnn/include/opencv2/dnn/dnn.hpp

@@ -93,6 +93,9 @@ CV__DNN_EXPERIMENTAL_NS_BEGIN
         DNN_TARGET_FPGA
     };
 
+    CV_EXPORTS std::vector< std::pair<Backend, Target> > getAvailableBackends();
+    CV_EXPORTS std::vector<Target> getAvailableTargets(Backend be);
+
     /** @brief This class provides all data needed to initialize layer.
      *
      * It includes dictionary with scalar params (which can be read by using Dict interface),