app: port GimpBlendTool to pause()/resume()-less drawing

Instead, keep around the created GimpCanvasItems, and update them when
the blend coordiates change. Add setters to GipmCanvasLine and
GimpCanvasHandle which take care of calling begin_change() and
end_change() on the items around the change, so thes invalidate
properly.

app/display/gimpcanvasline.h

@@ -55,5 +55,11 @@ GimpCanvasItem * gimp_canvas_line_new      (GimpDisplayShell *shell,
                                             gdouble           x2,
                                             gdouble           y2);
 
+void             gimp_canvas_line_set      (GimpCanvasLine   *line,
+                                            gdouble           x1,
+                                            gdouble           y1,
+                                            gdouble           x2,
+                                            gdouble           y2);
+
 
 #endif /* __GIMP_CANVAS_LINE_H__ */