provide option to load models from KMFs directly

[Trass3r]

#409 (comment)
#409 (comment)

The ModelViewer class already does this when directly given a kmf file:

OpenKeeper/src/toniarts/openkeeper/tools/modelviewer/ModelViewer.java

Lines 259 to 265 in d21dd2a

	if (kmfModel != null) {
	try {
	KmfFile kmf = new KmfFile(kmfModel);
	KmfModelLoader loader = new KmfModelLoader();
	KmfAssetInfo asset = new KmfAssetInfo(assetManager, new AssetKey(kmfModel.toString()), kmf, false);
	Node node = (Node) loader.load(asset);
	setupModel(node, false);

But not when loading from the list of models.
Would also be helpful for development to skip the whole conversion process on every iteration.

[Trass3r]

@tonihele Maybe it should simply fall back to loading the KMF if the converted model is missing (and the cache is considered to be "up-to-date").
That would allow deleting them locally for faster iteration on the implementation.

One could also allow other model formats like gltf with the same model basename.
That would be more useful for asset replacement than .j3o for the reasons laid out in #409 (comment).
Should probably be combined with a kmf->gltf exporter. The .j3o is not useful in general it seems. Also when it comes to speeding up the loading.