[github-actions]

TiledMap.spawn("Objects") creates a sprite for each object of a layer whose
type (Tiled's "class") is the name of a sprite class, puts it where the object
is and adds it to the stage. Objects drawn as areas give their sprite that area
as its hitbox. The sprite class needs a constructor taking the MapObject, one
taking (double x, double y), or one without parameters.

A sprite that only draws with its pen no longer shows the red "?" marker for a
missing costume. The marker (and its warning) now appears only for a sprite that
stays without a costume, a hitbox and a used pen for a second.

Editors can show a preview of a stage: started with
-Dscratch4j.preview=<folder>, a program draws the first frame of its stage,
saves it as frame.png with the sprites described in stage.json, and ends.
-Dscratch4j.preview.stage=<Class> previews another stage, and
-Dscratch4j.preview.omit=<fields> leaves the sprites in those fields out of
the picture.

💥 BREAKING CHANGE: A stage no longer opens the window itself. Create the window
first and put the stage into it:

Window myWindow = new Window(480, 360);
myWindow.setStage(new MyStage());

new Stage(width, height) and new Stage(width, height, assets) are gone: a
stage is always as big as the window it is shown in, so the size (and the assets
folder) go to new Window(...). Remove super(width, height) from your stages.
A stage made before there is a window says how to fix it, and a window nobody
put a stage into says so too.