API Additions
* Add IteratorGenerator.
* Add HiddenInput (<input type="hidden">), FileInput (<input type="file">),
SearchInput (<input type="search">), and TimeInput (<input type="time">).
* Add LineBreak (<br>).
* Table now has two overridable generator methods generate_header_rows() and
generate_rows().
* Add TextArea.placeholder property.
* Add NumberInput.number property.
* Add Form.target property and Form.set_blank_target().
* Add data_attribute(), list_html_attribute() and time_html_attribute().
API-Incompatible Changes
* Improve Element.id handling and raise ValueError on invalid ids.
* The default name of all input elements has been changed from None to the
empty string to match Input.name.
* NumberInput constructor: Replace value argument by number.