Warning about image layer image loaded from .tmj format

[bjorn]

The docs at https://dcronqvist.github.io/DotTiled/docs/essentials/loading-maps.html have a warning:

Using the .tmj file format will result in Image (the source image for image layers) not having the same amount of information as for the .tmx format.

Assuming this warning is about the missing imagewidth and imageheight properties on image layers, these have been added in mapeditor/tiled@06e94bd (Tiled 1.11.1). If DotTiled reads those properties, this warning could be removed.