Implement setActive in ol.interaction.Draw

This PR fixes two bugs in the draw interaction:

* Currently `abortDrawing_` doesn't clear the feature overlay if `this.sketchFeature_` is `null`. This means that, when the interaction is used to draw lines or polygons , the initial point (the one drawn on the feature overlay before the sketch feature is created) is not removed from the overlay. 0f82687 fixes this by always clearing the feature overlay when `abortDrawing_` is called.

* Currently `setActive(false)` does not abort the drawing. 1f005d fixes it by overloading `setActive` and taking specific action in that function.

Please review.