Bug 592003 - Shift+click should always modify selection

Shift-click inside an existing selection reduces the selection to the
range from the insert mark to the clicked point instead of removing the
selection. This makes GtkTextView more consistent with GtkEntry.

gtk/gtktextview.c

@@ -4314,7 +4314,8 @@ gtk_text_view_button_press_event (GtkWidget *widget, GdkEventButton *event)
 
           if (gtk_text_buffer_get_selection_bounds (get_buffer (text_view),
                                                     &start, &end) &&
-              gtk_text_iter_in_range (&iter, &start, &end))
+              gtk_text_iter_in_range (&iter, &start, &end) &&
+              !(event->state & GDK_SHIFT_MASK))
             {
               text_view->drag_start_x = event->x;
               text_view->drag_start_y = event->y;