Fix BL-786: cursor half out of the box, typing does nothing

The problem is a regression; we previously had a workaround that was
causing a different problem and was recently removed. This time around
we insert an empty <span> to give gecko some bearings so it doesn't
focus on our language qtip. We then remove the span when leaving the
box.
Merge pull request #173 from BloomBooks/BL-786TextFocusProblem

Fix BL-786: cursor half out of the box, typing does nothing
Fix BL-777, where the languagetip was showing in the wrong edit box.

A previous workaround for 2 gecko bugs worked like this: If the focus
is in the text box, hide :after tag that was showing the language name,
and instead add a separate div inside the translation group. But in
doing so, it had no way of being in the right place
for bi and tri-lingual modes.

Due to a previous fix (PR#173), this is all unneeded now, and this
commit just takes us back to just relying on the css-positioned
language tag.
Fix BL-744: Javascript error the first time a user modified a style

Our previous code created the necessary element in <head>, but in the
xulrunner 29 we're using, the element we added isn't a Stylesheet
object. So this fix gets the new stylesheet object from the
document.styleSheets array after creating that new element,
and then all is well.
Merge pull request #174 from BloomBooks/BL-777LanguageTagMovesToWrongBox

Fix BL-777, where the languagetip was showing in the wrong edit box.
Merge pull request #175 from BloomBooks/BL-744ErrorOnFirstStyleChange

Fix BL-744: Javascript error the first time a user modified a style