Add temporary simpler Tx scheduler for WSPR-LF

Until the advanced band hopping scheduler is upgraded to work with T/R
periods other  than 2  minutes this scheduler  uses a  strictly random
scheduler, except  that Tx percentages  less than 40 defer  Tx periods
that would otherwise be consecutive.

git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@7687 ab8295b8-cf94-4d9e-aec4-7959e3be5d79

mainwindow.cpp

@@ -6078,7 +6078,7 @@ void MainWindow::WSPR_scheduling ()
     band_hopping_label.setText (hop_data.period_name_);
   }
   else {
-    m_WSPR_tx_next = m_WSPR_band_hopping.next_is_tx ();
+    m_WSPR_tx_next = m_WSPR_band_hopping.next_is_tx ("WSPR-LF" == m_mode);
   }
 }