[ticket/12093] Check that document.selection is supported.

Inserting text at the caret position in IE11 now works as in other browsers (storeCaret() isn't executed when inserting bbcode/smilies or a quote) - setting textEl.caretPos is no longer necessary, so simply checking that document.selection is supported should suffice.

http://tracker.phpbb.com/browse/PHPBB3-12093