Smarter layer/group toggling

[jng]

Each check/uncheck of a layer/group in a Fusion legend fires off a refresh request. If you're checking/unchecking a series of layers/groups in quick succession, it will send off a request for each item checked/unchecked!

We should be a bit smarter about this. Instead of immediately sending off a refresh request on check/uncheck, we should schedule a call to refresh via setTimeout(). Each subsequent check/uncheck, should see if we have a scheduled refresh and if we do, reset the timeout on the refresh call.

This way, we can rapidly check/uncheck a series of layers/groups and only have one refresh call sent back.

[jng]

Fixed r2806