#794: Initial support for multiple constraints and alerts

Multiple error constraints work. Other constraints also work on the
server but do not yet display properly in the UI.

- introduce constraint levels
- nested xf:constraint elements with levels
- support for multiple xf:alert able to bind to levels and constraints
- statically
  - gather multiple constraints
  - gather multiple alerts per control
- dynamically
  - store all failed constraints into BindNode
  - dependencies check all MIPs for the given name/level
  - xxforms-constraints-changed event
  - runtime algorithm to determine active alerts
  - force dirty alerts when needed
- also
  - improve dispatch of change events
  - XForms schema updates
  - refactoring and renaming