Handle intentional blur

That is, if the typist deliberately clicks outside an editable field, the selection should clear and any ghost parens should solidify; but if the typist switches tabs/windows and then switches back, nothing should change, which should be possible: http://www.one-tab.com/page/sLCTvblJSfqXEzZVL4Elmw

Previous discussion: https://github.com/mathquill/mathquill/pull/262#issuecomment-37904712