Adding ModifyFeature interaction

This is a first draft. The way geometry changes are handled for
now is a bit clumsy. Both updating the feature cache RTree and
making the layer aware of feature and geometry changes could be
handled in a smarter way if these changes would be made through
the layer instead of directly on the geometry or feature.

src/ol/layer/vectorlayerrenderintent.js

@@ -6,6 +6,7 @@ goog.provide('ol.layer.VectorLayerRenderIntent');
  */
 ol.layer.VectorLayerRenderIntent = {
   DEFAULT: 'default',
+  FUTURE: 'future',
   HIDDEN: 'hidden',
   SELECTED: 'selected',
   TEMPORARY: 'temporary'