function getTMX(elt) {
	// force as string
	elt = "" + elt;
	if (elt in tmxList) {
		return tmxList[elt];
	}
	return null;
}
return the specified TMX/TSX object
@memberof loader
@param {string} elt - name of the tmx/tsx element ("map1");
@returns {object} requested element or null if not found