Implement resource loading.

Now the AXRDocument is the one that deals with the axr:// scheme.
Fix typo on property name when getting the end iterator, when removing a child from a container
Implement loading of xml files into a target node, using @request's target property
Implement printing to strings on selections
Add convenience function for easier debugging
Whitespace fixes
Make propertiesSize() function const
Store observers in lists of HSSObservableMappings, so that they get notified in the same order as they were added
Improve the sanitization code