This is now an instance method, whereas before it was global. Annoyingly, we
need to access this from from within fixCursor which has no reference to the
RTE instance itself (and it would be a pain to pass one down). For now, just
referring to the global `editor` variable if it exists (i.e. if the script
loaded in an iframe). Need a better solution longer term though.
* If you load the squire.js script into a top-level page rather than an iframe,
it will add a Squire constructor to the global scope.
* The Squire constructor can be used to instantiate multiple instances on the
same page without having to load/parse/execute the full code every time.
* For each instance, create a new iframe, then call `new Squire( document )`,
with the document node for each iframe.
Won't throw an error if called on a node that is not part of a document, but
will now instead return the path from whatever the root element is down to the
node.
* Do not move range boundaries inside images; these should always be treated as
leaf nodes.
* Workaround FF bug where it may return range as being inside of image node.
If the node containing the zero-width space character was merged by a call to
Range#mergeInlines, the character was not being removed, making for odd cursor
movement.
