WMTS overlays

In the tree we can have a level one LayerGroup like this:

 {
   "isExpanded": false,
   "isInternalWMS": false, // false is requied to contains WMTS layers
   "name": "Test WMTS",
   "children": [ ... ]
 }

A child is something like this:

 {
    "type": "WMTS", // required
    "url": "http://wmts.geo.admin.ch/1.0.0/WMTSCapabilities.xml", // capabilities URL
    "identifierAttribute": null, // same as other layers
    "no2D": false, // same as other layers
    "id": 999, // same as other layers
    "minResolutionHint": 0.0, // same as other layers
    "maxResolutionHint": 100.0, // same as other layers
    "isChecked": true, // same as other layers
    "isVisible": true, // same as other layers
    "legend": true, // same as other layers
    "legendURL": "", // empty => get metadata url from capabilities
    "metadataURL": "", // empty => get metadata url from capabilities
    "mapserverLayers": "toto", // related mapserver layer (for print and query)
    "name": "ch.are.gemeindetyp-1990-9klassen", // name of the WMTS layer throw OpenLayers.i18n => display name 
    "matrixSet": "21791", // WMTS matrix set
    "dimensions": {"test": "toto"}, // WMTS dimensions
    "style": "", // WMTS style, empty => default
 }

OpenLayers PullRequest that can helps: https://github.com/openlayers/openlayers/pull/271