improved quality of handling model event bindings when they are passes as a params of widgets:
* added saving and cleaning subscriptions when widget is destroyed
* added support for replacing one model value with another with correct subscription handling
* fixed absence of event subscribing in particular situations
Implemented quite hakky protection against redundantly escalating of pushing context changes and re-rendering child widgets in case of parent widget is going to re-render itself. Introduced two things:
* mechanism of "sentencing to death" of the children widgets, and checking if the widget is sentenced before reacting to any events
* (this is very doubtful) firing of push binding events even when the value is set to :deferred special value