put the chunk types into the chunk lists, so that we can selectively disable drag targets
Cancel drops into invalid targets. Doesn't yet disable invalid targets.
Depends on the CSS classes introduced in 848286aabe1cfcd2317a75bed97c7289ef076360
This might be necessary to properly disable/enable sortables inside start.
http://stackoverflow.com/a/21728218
Builds on 94e47a4013d0e8ce15e18e1c91f8c07774ac3d40 and 848286aabe1cfcd2317a75bed97c7289ef076360