Copy-paste data fixes

* Assign cut/copy/paste events to the document so that
  they are triggered when the CE doesn't have real focus
  e.g. only a FocusableNode is selected
* Use clipboardData.setData when available. The key can
  be stored and retrieved perfectly in text/xcustom but
  this technique doesn't work in FF or IE
* Just use text content for key as this is what is written
  to the clipboard, so this can be used directly without
  having to get the content from the pasteTarget.

Bug: 48604
Bug: 49396
Change-Id: Ib3702f9441f6ee3fa34ec071f00994dd7e591d99
Merge "Copy-paste data fixes"