Implement #1080 "xxf:split() function"

Also refactored xxf:classes() and xxf:invalid-binds() to use new functions in FunctionSupport.
Fix #1085 "Bind XPath analysis incorrect modifies expressions"

Also caused #1086.
Refactor: simplify functions
For #794: fr:error-summary keeps track of all alert levels

- more context information for xxforms-constraints-changed
- listen to xxforms-constraints-changed
- store alert levels
- style warning / info differently from error
- FB updates to reflect error/warning/info
fr:error-summary: remove unneeded handling of xforms-value-changed