LayerTree WMTS improvements

This PR aims to address both https://github.com/camptocamp/sitn_c2cgeoportal/issues/110 and https://github.com/camptocamp/cgxp/issues/370. More specifically, this PR adds loading spinner to WMTS group nodes, just like WMS. It also makes it so one WMTS GetCapabilities per WMTS server, as opposed to per WMTS layer, is issued. Commit 5e8408d also slightly simplifies the way layer indices are managed.

Please review.