Moving toward multiplier and dupe detection for contest modes

There's not  much to show  for this so far  but some of  the necessary
infrastructure is in place.

widgets/FrequencyDeltaLineEdit.cpp

@@ -0,0 +1,54 @@
+#include "FrequencyDeltaLineEdit.hpp"
+
+#include <limits>
+
+#include <QDoubleValidator>
+#include <QString>
+#include <QLocale>
+
+#include "moc_FrequencyDeltaLineEdit.cpp"
+
+namespace
+{
+  class MHzValidator
+    : public QDoubleValidator
+  {
+  public:
+    MHzValidator (double bottom, double top, QObject * parent = nullptr)
+      : QDoubleValidator {bottom, top, 6, parent}
+    {
+    }
+
+    State validate (QString& input, int& pos) const override
+    {
+      State result = QDoubleValidator::validate (input, pos);
+      if (Acceptable == result)
+        {
+          bool ok;
+          (void)QLocale {}.toDouble (input, &ok);
+          if (!ok)
+            {
+              result = Intermediate;
+            }
+        }
+      return result;
+    }
+  };
+}
+
+FrequencyDeltaLineEdit::FrequencyDeltaLineEdit (QWidget * parent)
+  : QLineEdit (parent)
+{
+  setValidator (new MHzValidator {-std::numeric_limits<FrequencyDelta>::max () / 10.e6,
+        std::numeric_limits<FrequencyDelta>::max () / 10.e6, this});
+}
+
+auto FrequencyDeltaLineEdit::frequency_delta () const -> FrequencyDelta
+{
+  return Radio::frequency_delta (text (), 6);
+}
+
+void FrequencyDeltaLineEdit::frequency_delta (FrequencyDelta d)
+{
+  setText (Radio::frequency_MHz_string (d));
+}