Geometry index won't be updated correctly if feature geometry is changed using `feature.setGeometry(new_geometry)`
ol.interaction.Snap should listen for a 'change:geometry' event on ol.Feature
More than one features can be be changed while handlingDownUpSequence
exampe: ol.interaction.Modify supports simultaneous editing of multiple features
When a geometry changes old indexed segments are searched by the feature extend and removed. New ones are added.
Old feature extent sholud be used when removing the old indexed segments.
This commit changes ol.Snap -> ol.interaction.Snap, which extends
ol.interaction.Pointer. The 'pointerdown', 'pointermove' and
'pointerup' map browser events are hanlded to edit the pixel and
coordinate properties to make them 'snap' to the closest feature.
