The Great [...] Rewrite of 2013: Clipboard edition

Firefox:
* Doesn't fire events without a selection, so when all we have is a
  focusable node, create a dummy selection in the paste target so the
  browser triggers an event on command+C.

Clipboard keys:
* Instead of just creating a text hash which may or may not match up
  with the plain text in the clipboard data, just convert the clipboard
  store into an array and use the index as the key. Also prepend a
  random number (clipboardId) so on paste we can verify the copy came
  from the same CE document.
  The text hash is now used for validation if only HTML is written
  to the clipboard.

HTML:
* Instead letting the browser grab CE HTML, always get DM HTML from
  the converter as it is cleaner (no phantoms, linefeed markers, extra
  class names etc.), and will make cross-ve-instance pasting easier
  in the future.

Clipboard overriding:
* Always override the clipboard with the above HTML, either using
  clipboardData.setData if it is available (webkit) or by throwing
  the HTML into the pasteTarget and selecting it during pre-copy.

Bug: 53375
Bug: 49396
Bug: 52096
Change-Id: Id0f39c10818047bb4b8922e97cead153a770757f
Merge "The Great [...] Rewrite of 2013: Clipboard edition"