special treatment for '' ligature in TeX, which counts as word separator

--- a/src/Tools/jEdit/src/spell_checker.scala	Sun Apr 13 19:20:03 2014 +0200
+++ b/src/Tools/jEdit/src/spell_checker.scala	Sun Apr 13 19:32:49 2014 +0200
@@ -138,6 +138,9 @@
     val result = new mutable.ListBuffer[Text.Range]
     var offset = 0
 
+    def apostrophe(c: Int): Boolean =
+      c == '\'' && (offset + 1 == text.length || text(offset + 1) != '\'')
+
     @tailrec def scan(pred: Int => Boolean)
     {
       if (offset < text.length) {
@@ -152,7 +155,7 @@
     while (offset < text.length) {
       scan(c => !Character.isLetter(c))
       val start = offset
-      scan(c => Character.isLetterOrDigit(c) || c == '\'')
+      scan(c => Character.isLetterOrDigit(c) || apostrophe(c))
       val stop = offset
       if (stop - start >= 2 && !check(text.substring(start, stop)))
         result += Text.Range(start, stop)