* The game's tick counter was moved from Doodle.ticks to shmem.Tick
where it is more easily available from every corner of the code.
* Fix a bug in the Level Editor where dragging an already-existing actor
from one part of your map to another, would cause it to lose all its
data (especially its UUID), breaking links to other doodads. Now the
existing Actor catches a ride on the drag object to be reinserted
later.
* Animate the Link Line visualizers between actors. They now animate a
blinking color between magenta and grey-ish.
* Add new pkg/drawtool with utilities to abstract away drawing actions
into Strokes and track undo/redo History for them.
* The freehand Pencil tool in EditorMode has been refactored to create a
Stroke of Shape=Freehand and queue up its world pixels there instead
of directly modifying the level chunker in real time. When the mouse
button is released, the freehand Stroke is committed to the level
chunker and added to the UndoHistory.
* UndoHistory is (temporarily) stored with the level.Level so it can
survive trips to PlayScene and back, but is not stored as JSON on
disk.
* Ctrl-Z and Ctrl-Y in EditorMode for undo and redo, respectively.
Noah