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/interaction/draginteraction.js

@@ -26,6 +26,12 @@ ol.interaction.Drag = function() {
    */
   this.dragging_ = false;
 
+  /**
+   * @type {number} Delta for INTERACTING view hint. Subclasses that do not want
+   *     the INTERACTING hint to be set should override this to 0.
+   */
+  this.interactingHint = 1;
+
   /**
    * @type {number}
    */
@@ -60,6 +66,14 @@ ol.interaction.Drag = function() {
 goog.inherits(ol.interaction.Drag, ol.interaction.Interaction);
 
 
+/**
+ * @return {boolean} Whether we're dragging.
+ */
+ol.interaction.Drag.prototype.getDragging = function() {
+  return this.dragging_;
+};
+
+
 /**
  * @param {ol.MapBrowserEvent} mapBrowserEvent Event.
  * @protected
@@ -115,7 +129,7 @@ ol.interaction.Drag.prototype.handleMapBrowserEvent =
       goog.asserts.assertInstanceof(browserEvent, goog.events.BrowserEvent);
       this.deltaX = browserEvent.clientX - this.startX;
       this.deltaY = browserEvent.clientY - this.startY;
-      view.setHint(ol.ViewHint.INTERACTING, -1);
+      view.setHint(ol.ViewHint.INTERACTING, -this.interactingHint);
       this.dragging_ = false;
       this.handleDragEnd(mapBrowserEvent);
     }
@@ -131,7 +145,7 @@ ol.interaction.Drag.prototype.handleMapBrowserEvent =
         (mapBrowserEvent.getCoordinate());
     var handled = this.handleDragStart(mapBrowserEvent);
     if (handled) {
-      view.setHint(ol.ViewHint.INTERACTING, 1);
+      view.setHint(ol.ViewHint.INTERACTING, this.interactingHint);
       this.dragging_ = true;
       mapBrowserEvent.preventDefault();
       stopEvent = true;