ExampleLevel: Make gas pillar cloud work

As of at least U8, gas pillar cloud actors now spawn clouds on each
gas pillar they know about. But this knowledge is set up inside the
editor when clicking the `Gather Nearby Pillar Locations` button in
the gas pillar cloud's details panel.

Now that `Gather Nearby Pillar Locations` has been implemented, use
it to make the gas cloud in ExampleLevel behave correctly. While at
it, also reorganise the poison gas actors to make the gas cloud the
parent. This should allow moving the pillars around individually.

Signed-off-by: Angel Pons <[email protected]>