Add option to specify entity picture

Previously it was possible to have a composite entity use the picture from one of its input entities. This release adds an option to use an image file instead, or for YAML configuration, an URL.

For an image file, it can either be one that is already on the system in the "/local" directory, which is actually <config_path>/www (typically /config/www.) Or, when using the UI to configure the composite entity, it is also possible to upload an image file (which will then be stored in "/local/uploaded".)