Extend TreeNodeLoading plugin for WMTS

This commit extends the TreeNodeLoading plugin to make it work with group nodes that do not have OpenLayers layers directly attached to them. This is necessary for groups of WMTS layers.
Register loadstart/end listeners on WMTS layers

When the WMTS GetCapabilities response is received we call registerLoadListeners on the TreeNodeLoading plugin to register loadstart and loadend listeners for the new WMTS layer.