Replace deprecated provides() with hasFormat()

QDropEvent::provides() has been deprecated from the move from Qt 3 to Qt 4.
Replace this instance with the hasFormat() method on the mimeData instance.

src/app/qgsbrowserdockwidget.cpp

@@ -76,7 +76,7 @@ class QgsBrowserTreeView : public QTreeView
 
       QTreeView::dragMoveEvent( e );
 
-      if ( !e->provides( "application/x-vnd.qgis.qgis.uri" ) )
+      if ( !e->mimeData()->hasFormat( "application/x-vnd.qgis.qgis.uri" ) )
       {
         e->ignore();
         return;