Add snapping to the Editing control

**To be reviewed**

The configuration has to be done [in the code](https://github.com/vicb/cgxp/compare/snapping?expand=1#diff-74cd57288c385713732357d9455121b1R72) by the integrator.

All editable layers snap to the targets configured in the code. A target layer layer could be a "single" layer or be loacted inside a map server layer group.

The snapping is active in creation and edition, when the target layers are both visible and in range.

Check [the demo](http://baselstadt-mapbs-dev.gis.internal/vberchet/wsgi/edit?debug=true): the snapping target is the "LAYEROBJ_AV_PARZELLEN" layer which is also present in the "ed_group" map server group. You must select the "Amtliche Vermessung" theme.

_Note: You have to be logged as "admin" to be able to see the editabled layers.