projectexplorer: added functionality for opening a folder in another tab/window

app/project/projectviewmodel.cpp

@@ -38,7 +38,7 @@ ProjectViewModel::ProjectViewModel(QObject *parent) :
   columns_.append(kRate);
 }
 
-Project *ProjectViewModel::project()
+Project *ProjectViewModel::project() const
 {
   return project_;
 }

app/project/projectviewmodel.h

@@ -65,7 +65,7 @@ public:
    *
    * Currently active project or nullptr if there is none
    */
-  Project* project();
+  Project* project() const;
 
   /**
    * @brief Set the project to adapt