* Use `go-bindata` to embed built-in doodads and levels directly into
the Doodle binary. `make bindata` produces the bindata source file.
* Add `FromJSON()` method to Levels and Doodads to load objects from
JSON strings in memory (for bindata built-ins or WASM ajax requests)
* Update file loading functions to check the embedded bindata files.
* pkg/config.go#EditFile:
* Supports editing a level from bindata (TODO: remove this support)
* If the "assets/levels/%(simple-name.level)" exists in bindata,
edits that drawing.
* No such support for editing built-in doodads.
* WASM has no filesystem access to edit files except built-in
levels (yet)
* pkg/doodads#ListDoodads:
* Prepends built-in doodads from bindata to the returned list.
* WASM: no filesystem access so gets only the built-ins.
* pkg/doodads#LoadFile:
* Checks built-in bindata store first for doodad files.
* WASM: tries an HTTP request if not found in bindata but can go no
further if not found (no filesystem access)
* pkg/filesystem#FindFile:
* This function finds a level/doodad by checking all the places.
* If the level or doodad exists in bindata built-in, always returns
its system path like "assets/doodads/test.doodad"
* WASM: always returns the built-in candidate path even if not found
in bindata so that ajax GET can be attempted.
* pkg/level#ListSystemLevels:
* New function that lists the system level files, similar to the
equivalent doodads function.
* Prepends the bindata built-in level files.
* WASM: only returns the built-ins (no filesystem support)
* Desktop: also lists and returns the assets/levels/ directory.
* pkg/level#LoadFile:
* Like the doodads.LoadFile, tries from built-in bindata first, then
ajax request (WASM) before accessing the filesystem (desktop)
* Menu Scene: TODO, list the built-in levels in the Load Level menu.
This feature will soon go away when WASM gets its own storage for user
levels (localStorage instead of filesystem)
* 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.
