Move the HUD to a UIComponent iframe

This PR
* moves the HUD to an iframe, with its interface provided by `UIComponent`
* updates `Tween` to keep track of its own timers and to modify CSS rather than directly changing an element's style.
* uses the HUD iframe to provide an input (a `<span contentEditable="plaintext-only">` element) for find mode, which
  - supports copy/paste
  - accepts characters which are generated from multiple subsequent keypresses
  - provides a caret, so the user can modify a search without re-typing it

It also includes a couple of simple fixes:
* A typo where `vimiumUIComponentShowing` should have been `vimiumUIComponentVisible`.
* A place where the update from `handledKeydownEvents` to `KeydownEvents` had missed.

This fixes #556.
Having `Tween` out of the front end is already an improvement.  No need to move it to the HUD frame, for now.