[Qt] allow to execute a callback during splashscreen progress

src/qt/bitcoin.cpp

@@ -578,6 +578,7 @@ int main(int argc, char *argv[])
     //   Need to pass name here as CAmount is a typedef (see http://qt-project.org/doc/qt-5/qmetatype.html#qRegisterMetaType)
     //   IMPORTANT if it is no longer a typedef use the normal variant above
     qRegisterMetaType< CAmount >("CAmount");
+    qRegisterMetaType< std::function<void(void)> >("std::function<void(void)>");
 
     /// 3. Application identification
     // must be set before OptionsModel is initialized or translations are loaded,