Implements basic form resetting

What can this do? Reset `<input type=text>` fields back to their default
value through a call to a form's reset method. That's all for now!

Fixes compile error after rebase
Adds reset method to FormControl
Implements FormControl for HTMLTextAreaElement
Implements reset button
htmltextarea: Fixed some value_changed issues

Also modified tests/html/textarea.html to allow for the testing of the
textarea's dirty value flag.
Implements DefaultChecked and resets of checkboxes
Implements dirty value/checked flags for input

And modifies test-inputs.html to test.

Fixes wpt breaking mistake
Updated WPT to expect newly passing tests
Implements :indeterminate pseudo-class

Addresses reviews