Move all Doodle source code into the src/ subpackage and move the
publicly shareable stuff into lib/, for example lib/ui and lib/render.
This cleans up the git root and helps make the Doodle UI library more
easily publishable as a separate open source project. Currently both
lib/ui and lib/render import one or two things from doodle/src that need
to be broken apart.
Adds the `doodad` binary which will be a command line tool to work with
Doodads and Levels and assist with development.
The `doodad` binary has subcommands like git and the first command is
`convert` which converts between image files (PNG or BMP) and Doodle
drawing files (Level or Doodad). You can "screenshot" a level into a PNG
or you can initialize a new drawing from a PNG.
