Don't untie a property when loading from xml

executing the fgcommand loadxml implicitely untied every previously
tied property if a property type was given in the property element.
That made it impossible to autosave aircraft data for tied properties.
This patch should fix this. It adds an inconsitency, though:
if a property with a given type is read in which has been tied to
before and the property types do not match, the property type in the
xml file will be ignored.