- Factor out some common code and move editor-specific code to a
dedicated js-file.
(Note!!! Can't factor out map-grid approach of editor, because the
cells containing Box2D bodies won't be on the same hierarchy level
(coordinate system) as the World, which is required by Box2D)

- Add a testing mode to allow for direct testing of an editor map.

- Load, save and maintain a whole level-set in the editor.

- Minor extensions to File API

- Add some dialogs.