WMTS support

This includes French GeoPortail WMTS support (discovering, viewing, changing styles & formats, querying, printing, getting unresolved legends ;-) ...

The layer browser was also refactored, and client-side libs updated to latest "near master" state.

IMPORTANT NOTES: 
 - the default MAP_SCALES were adjusted in order to match those from the GeoPortail. If you experiment tiles mis-alignment, please have a look at your server scales.
 - WMTS layers cannot be saved in a WMC. As a result, users accustomed to recovering their latest context might be surprised.