LayerGroups - advanced sync with visibility & opacity

[klokan]

All the layers added and removed in LayerGroups hierarchy are now synchronized via basic #27.

This ticket is about more advanced implementation which would allow to sync changes on visibility / opacity / brightness / contrast on the whole group affecting the individual layers and other LayerGroups inside of a particular group.
Cesium does not have concept of group layers, so the OL3Cesium wrapper would need to calculate the changes appropriatelly for all individual layers - and listen on additional changes.

Related is prevention of sync in the nested groups mentioned in #88 and comments of @gberaudo from this PR.

Related is also #19 with a performance trick when the root LayerGroup is hidden to stop rendering in OL3