Attr model, write protected attrs

* Added ability to forbid writing into attribute values from outside
Page class

* Merge page container functions into renderer
* Dropped deprecated page class
Forms3, time picker

* Time picker takes default value from input
Forms3, errors

* Displayed form errors are red by default
Form class

* Added input getter
* Object attrs use attr model
Forms, container

* Container add_element returns reference to the object
* Added container is_valid() method that runs is_valid() on all children
Forms, input class

* Dropped deprecated get_opts() method from input class
* Added basic validation for input class
Forms, renderer

* Added render_error_list method
Forms, widget class

* Added basic validation for widgets
* Added model validation for widgets
* Reserved ident 'action' for action widget
Forms, action widget

* Fixed some functionality for action widget
Forms, image widget

* Image widget renders errors