Fix #1252 "Strip unneeded whitespace in XForms processing"

- configurable WhitespaceXMLReceiver
  - one instance hooked before annotator
  - one instance hooked after extractor
  - configuration uses CSS selectors in properties
- annotator
  - don't send content of bound elements to template receiver
  - some refactoring and use a stack
- extractor
  - ability to directly output root element for XBL trees
- template trees use SAXStore
- unit tests properties include base and XForms properties