first take at MXML Binding implementation

* only get functions are checked for [Bindable(event="...")] annotations for now
* when using such a property in a binding expression {...}, it is
  dependency-tracked and the expression is re-evaluated when it
  changes
* when using a config object, the config property is assigned by evaluating
  the binding expression right-hand side (source) once, all subsequent invalidations
  trigger a set on the target object.
* on the fly, made generated event listener methods private and improved naming scheme
* also change joo.addEventListener() signature so that optional eventType comes last
(Ext) default change event are lower case
look for "id" attribute first to determine varName
better way of generating property assignment code and binding execution:

* for config options, evaluate right-hand side once without dependency tracking
* only establish Binding if target property setter is defined as an [Accessor]
* execute Bindings late, after the whole object structure has been fully instantiated

Thus
* got rid of ConfigClass detection (their properties are never [Accessor]s anyway)
* got rid of special mode in Binding#execute(true) to return evaluated right-hand
  side immediately. Instead, MXML-generated code reuses the evaluation function.