Active layer group

[ychyss]

I notice that src/gui/stage.js return the active layergroup by getActiveLayerGroup. But I cannot change the current active one and it don't fire the events like 'active-lg-change'...

I use multi layergroups in my project. But I cannot notify the control buttons when user change into different layergroups. They display different data, so I need different status of my buttons.

Could you consider about that? some function like setActiveLayergroup and event loops. :-)

[ivmartel]

Seems reasonable, check out #1455 for progress.