Improvements on the Form component

- ValueAttribute::getValue() now always returns a string
- ValueAttribute::getValueOrNull() is added to provide an equivalent functionality to NullFilter
- NullFilter is now definitely gone
- NULL values are now automatically replaced with empty string or array before populating a component
- Input\Submit::isClicked() is now called isPressed()
- Button\Submit now has an isPressed() method like Input\Submit
- Input\Hidden is now populated when submitting the form
- Select\SingleSelect now enforces that only one option can be selected at a time