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.

examples/modify-features.js

@@ -0,0 +1,88 @@
+goog.require('ol.Map');
+goog.require('ol.RendererHint');
+goog.require('ol.View2D');
+goog.require('ol.interaction');
+goog.require('ol.interaction.Modify');
+goog.require('ol.interaction.Select');
+goog.require('ol.layer.Tile');
+goog.require('ol.layer.Vector');
+goog.require('ol.parser.ogc.GML_v3');
+goog.require('ol.source.MapQuestOpenAerial');
+goog.require('ol.source.Vector');
+goog.require('ol.style.Fill');
+goog.require('ol.style.Rule');
+goog.require('ol.style.Shape');
+goog.require('ol.style.Stroke');
+goog.require('ol.style.Style');
+
+var raster = new ol.layer.Tile({
+  source: new ol.source.MapQuestOpenAerial()
+});
+
+var vector = new ol.layer.Vector({
+  id: 'vector',
+  source: new ol.source.Vector({
+    parser: new ol.parser.ogc.GML_v3(),
+    url: 'data/gml/topp-states-wfs.xml'
+  }),
+  style: new ol.style.Style({
+    rules: [
+      new ol.style.Rule({
+        filter: 'renderIntent("selected")',
+        symbolizers: [
+          new ol.style.Fill({
+            color: '#ffffff',
+            opacity: 0.5
+          }),
+          new ol.style.Stroke({
+            color: '#6666ff',
+            width: 0.5
+          })
+        ]
+      }),
+      new ol.style.Rule({
+        filter: 'renderIntent("temporary")',
+        symbolizers: [
+          new ol.style.Shape({
+            fill: new ol.style.Fill({color: '#bada55'}),
+            size: 16
+          })
+        ]
+      }),
+      new ol.style.Rule({
+        filter: 'renderIntent("future")',
+        symbolizers: [
+          new ol.style.Shape({
+            fill: new ol.style.Fill({color: '#013'}),
+            size: 16
+          })
+        ]
+      })
+    ],
+    symbolizers: [
+      new ol.style.Fill({
+        color: '#ffffff',
+        opacity: 0.25
+      }),
+      new ol.style.Stroke({
+        color: '#6666ff'
+      })
+    ]
+  })
+});
+
+var selectInteraction = new ol.interaction.Select({
+  layerFilter: function(layer) { return layer.get('id') == 'vector'; }
+});
+
+var map = new ol.Map({
+  interactions: ol.interaction.defaults().extend(
+      [selectInteraction, new ol.interaction.Modify()]),
+  layers: [raster, vector],
+  renderer: ol.RendererHint.CANVAS,
+  target: 'map',
+  view: new ol.View2D({
+    center: [-11000000, 4600000],
+    zoom: 4
+  })
+});