Disable undo/redo for the decoded text windows as we never use it and don't need the overhead

Thanks to Alex, VE3NEA, for this contribution.

git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@8594 ab8295b8-cf94-4d9e-aec4-7959e3be5d79

displaytext.cpp

@@ -17,6 +17,7 @@ DisplayText::DisplayText(QWidget *parent)
   , erase_action_ {new QAction {tr ("&Erase"), this}}
 {
   setReadOnly (true);
+  setUndoRedoEnabled (false);
   viewport ()->setCursor (Qt::ArrowCursor);
   setWordWrapMode (QTextOption::NoWrap);