Fold libopenshot_protobuf into libopenshot

- Move .proto files to src/ dir (not src/protobuf_messages)
- Generate protobuf files in src/CMakeLists.txt and include in
  'openshot' library target
- Don't install generated *.pb.h headers, which are now private

src/CVObjectDetection.cpp

@@ -16,6 +16,8 @@
 #include <iostream>
 
 #include "CVObjectDetection.h"
+#include "objdetectdata.pb.h"
+
 #include <google/protobuf/util/time_util.h>
 
 using namespace std;
@@ -450,8 +452,8 @@ bool CVObjectDetection::_LoadObjDetectdData(){
         const google::protobuf::RepeatedPtrField<pb_objdetect::Frame_Box > &pBox = pbFrameData.bounding_box();
 
         // Construct data vectors related to detections in the current frame
-        std::vector<int> classIds; 
-        std::vector<float> confidences; 
+        std::vector<int> classIds;
+        std::vector<float> confidences;
         std::vector<cv::Rect_<float>> boxes;
         std::vector<int> objectIds;