IE10 compat: Don't use dummy element to capture paste

IE10 refuses to paste into the dummy element directly.  Instead, capture
the initial selection range and the post-paste range, then extract the
content into the dummy element for cleaning before returning it to the
editor.
Rewrite for better browser compatibility

IE10 didn't play nice with the previous approach.  The actual editor is
now used for the paste interception, with the original content temporarily
removed.