* 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.
* Load SDL2 fonts from go-bindata storage so we don't have to ship
external font files on disk.
* Dedupe names of doodads so we don't show double on the front-end
(go-bindata bundled doodads + those on local filesystem)
* Use go-bindata for accessing wallpaper images.
* Better flashed messages walking you through the Link Tool.
* Stylize the title screen (MainScene) by rendering a live example level
as the background wallpaper, with mobile doodads in motion.
* Refactor the event system in the WASM render engine to serialize the
async JavaScript events into a channel, so that queued events are read
off serially in the main loop similar to SDL. This fixes keyboard
input issues, altho if you type really fast some input keys get lost.
