Add option to load geotagged image via RC into existing image layer

[jose1711]

Currently whenever a user executes https://josm.openstreetmap.de/wiki/Help/RemoteControlCommands#open_file giving a path to an image file as a parameter a new image layer is created in JOSM. Personally I would welcome an option that would only do this if there was no previous image layer available. By default the image would be added to an existing image layer (e. g. the one with the lowest index if there are multiple found).

[Don-vip]

This ImageLoader block should support reuse of existing image layer by passing new arguments to ImageLoader constructor:

        if (Boolean.TRUE.equals(PROP_ONE_LAYER_PER_FOLDER.get())) {
            entries.entrySet().stream().map(e -> new GeoImageLayer(e.getValue(), gpxLayer, e.getKey())).forEach(layers::add);
        } else {
            layers.add(new GeoImageLayer(entries.values().stream().flatMap(List<ImageEntry>::stream).collect(Collectors.toList()), gpxLayer));
        }

through these stacks:

	GeoImageLayer.create(Collection<File>, GpxLayer) line: 245	
	ImageImporter.importData(List<File>, ProgressMonitor) line: 111	
	ImageImporter(FileImporter).importDataHandleExceptions(List<File>, ProgressMonitor) line: 152	
	OpenFileAction$OpenFileTask.importData(FileImporter, List<File>) line: 394	
	OpenFileAction$OpenFileTask.realRun() line: 350	
...
	OpenFileAction.openFiles(List<File>, Options...) line: 127	
...
	OpenFileHandler.handleRequest() line: 56