Stashed data file can be dropped to model view

* utility functions
* style changes / renamings / slots -> normal functions
* extra flag to enable file drop (and changed signatures)
* split view load function into 2 parts, one can use passed file parameter

src/blackgui/dropsite.h

@@ -67,11 +67,10 @@ namespace BlackGui
         //! \copydoc QWidget::dropEvent
         virtual void dropEvent(QDropEvent *event) override;
 
-    private slots:
-        //! Style has been changed
-        void ps_onStyleSheetsChanged();
-
     private:
+        //! Style has been changed
+        void onStyleSheetsChanged();
+
         //! Clear
         void resetText();