Have embedders send DOM keys to servo and use a strongly typed KeyboardEvent
from the W3C UI Events spec. All keyboard handling now uses the new types.
Introduce a ShortcutMatcher to recognize key bindings. Shortcuts are now
recognized in a uniform way.
Updated the winit port.
Updated webdriver integration.
part of #20331
Supports sending keys to an element. The specification here is still
rather unfinished so the error handling and so on in this code will
need iteration as it becomes clearer what the expected behaviour is.