UTFGrid: remove OpenLayers.Layer.UTFGrid dependency 

By moving the `getFeatureInfo` function to `OpenLayers.Control.UTFGrid` and by changing the `findLayers` logic a bit (7bb96f7ec2cf149c27cf96f0fb323c23a2f8150c).

This change allows to use a WMTS as grid source (see #691).

Comments welcome.