file: the name of the file to save, relative to the current
directory (defaults to "fgfs.sav").
+loadxml - load XML file into property tree
+ filename: the path & filename of the file to load
+ targetnode: the target node within the property tree where to store the XML
+ file's structure
+
+savexml - save property tree node to XML file
+ filename: the path & filename for the file to be saved
+ sourcenode: the source node within the property tree where the XML file's
+ structure is assembled from.
+
panel-load - (re)load the 2D instrument panel
path: the path of the XML panel file, relative to $FG_ROOT (defaults
to the value of /sim/panel/path if specified, or