Update sceditor to latest version

This PR takes the changes made to SCEditor 1.3.7 (or whatever version we currently have in 2.1) and applies them to the latest version (1.4.3 as of this PR). A complete list of changes applied is available at http://smfgit.oldiesmann.us/index.php?topic=4.0. A brief test on my forum indicates that this should work fine, but I'd like others to test it as well before we pull this in, especially since three blocks of code which we modified in jquery.sceditor.bbcode.js weren't found in the new version.

I have not made any changes to SMF itself for this, so other tweaks may be required as well.

Ref: #93

Signed-off-by: Michael Eshom <oldiesmann@oldiesmann.us>