cylc gui graph view: crop proxy points

This closes #1115.

In `cylc gui`'s graph view, proxy nodes are automatically cropped if all the 
nodes in their cycle point are proxies.

This helps with #1115, although you may still get task proxies in the first one 
or two cycle points sticking around because their cycle points contain immortal 
"succeeded" tasks.

@hjoliver, please test (to see if the behaviour is what we want) and review.