WMTS overlay are processed as WMS layers when not in a layer group

(committed on behalf of @sbrunner )

When a WMTS overlay is not embedded within a layergroup (ie. directly nested at the theme root), a WMS GetMap request is done instead of using WMTS.

This PR makes sure WMTS is actually used when the overlay is not in a layer group.

There is still a problem to fix: the opacity slider of the WMTS layer is at "opacity: 100%" whereas the slider cursor is positioned at the left end of the slider (opacity: 0)...