New feature: link a Start Flag to another doodad in your level
and you will play as that doodad instead of Boy. All Creatures
are designed to be playable. Playing as "other" doodads leads
to interesting effects, like not being able to activate buttons,
switches, or warp doors and not having an inventory to pick up
keys. The Anvil is fun: it can destroy other mobile doodads by
jumping on them.
If the actor does not specify that it has gravity, the gameplay
starts in antigravity mode. This will be the vast majority of
non-mobile doodads and the Bird.
Other changes:
* The Blue and Red Azulians now share a doodad script.
* The Azulians AI is still to walk back and forth, pickup keys and
press buttons. The Blue Azulian walks slower than the red one.
* The Blue Azulian is no longer hidden from the doodads list.
* Actor UUID values in levels are now V1 UUIDs (time-ordered).
This will help to reliably resolve conflicts in draw order
of overlapping doodads (newest added to level wins).
* Link Tool: clicking on a pair of already-linked doodads will
now unlink them, so you don't have to delete one to delete
the link.
* Actor Tool: deleting an actor immediately calls PruneLinks()
to clean up any links that the deleted doodad might have.
* On the Doodads tab is the Link button to enter the Link Tool.
* Click Link, then click the 1st doodad on the level, then click the 2nd
doodad to complete the link.
* The actors struct in the Level holds the link IDs for each actor.
