DragBox control provides visual shiftdragzoom feedback

Adding a new DragBox control and using it in the shiftdragzoom interaction to provide visual feedback of the zoom box. The control is nicely separated from the interaction - it only draws the box and does not perform any action.

css/ol.css

@@ -8,6 +8,11 @@
   -webkit-tap-highlight-color: rgba(0,0,0,0);  
 }
 
+.ol-dragbox {
+  position: absolute;
+  border: 2px solid red;
+}
+
 .ol-renderer-webgl-canvas { width:100%; height:100%; }
 
 .olTile {