GitLab

Add plumbing for the most basic player:

- A `Player` component to mark the entity which is the player.
- A `Player` resource which holds the entity ID of the player.
- A `draw_tmp_player_system` to draw the player as a circle (for now).
- A `create_player` function to create the player entity.
- Update the `moving_system` to put the camera at the player position.

Due to the camera translation, the clear is currently not actually
happening over the entire visible canvas.  Reset the transform before
clearing to ensure it's working correctly.

Just as an example to build on.

It's only relevant for mouse events so only make it available there.

Don't ever leak the handlers, keep track of them and drop them when they
are no longer needed.

We currently have a `state` and a `states` module.  This is confusing,
rename the state module to `gamestate` to make a more obvious
distinction.

Copy the constants from www/src/_rc3-colors.scss to a rust module as
well.

The camera system sets the transform of the canvas.  The pixel coordinate of
the camera position is centered in the canvas.