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/objectliterals.jsdoc

@@ -377,6 +377,14 @@
  * @todo stability experimental
  */
 
+/**
+ * @typedef {Object} ol.interaction.ModifyOptions
+ * @property {undefined|function(ol.layer.Layer):boolean} layerFilter Filter
+ *     function to restrict modification to a subset of layers.
+ * @property {number|undefined} pixelTolerance Pixel tolerance for considering
+ *     the pointer close enough to a vertex for editing. Default is 20 pixels.
+ */
+
 /**
  * @typedef {Object} ol.interaction.SelectOptions
  * @property {ol.events.ConditionType|undefined} addCondition A conditional