...and shadow tree with TextNode for "value" attribute is created.
This means InlineFormattingContext is used, and button's text now
respects CSS text-decoration properties and unicode-ranges.
This creates a button to prompt users to select a file, and a label to
show information about the selected file(s). Clicking either shadow
element will activate the input element.
The setter was missing an implementation for the default and default/on
value attribute modes. This patch adds a method to get the current value
attribute mode, and implements the value setter and getter based on that
mode according to the spec.
