Start implememntation of database based Fox log model and a new Fox log window widget

This change incorporates a reorganization of the GUI code with
widgets, validators, models, and item delegates being moved to
sub-directories.

Relax  the   requirements  of   the  ForeignKeyDelegate   and  related
CandidateKeyFilter classes to  allow them to work  with constant model
pointers for both referenced and referencing models.

validators/MaidenheadLocatorValidator.cpp

@@ -0,0 +1,73 @@
+#include "MaidenheadLocatorValidator.hpp"
+
+MaidenheadLocatorValidator::MaidenheadLocatorValidator (QObject * parent, Length length, Length required)
+  : QValidator {parent}
+{
+  switch (length)
+    {
+    case Length::field:
+      re_.setPattern ({R"(^(?<field>[A-Ra-r]{2})$)"});
+      break;
+    case Length::square:
+      if (Length::field == required)
+        {
+          re_.setPattern ({R"(^(?<field>[A-Ra-r]{2})([0-9]{2}){0,1}$)"});
+        }
+      else
+        {
+          re_.setPattern ({R"(^(?<field>[A-Ra-r]{2})[0-9]{2}$)"});
+        }
+      break;
+    case Length::subsquare:
+      if (Length::field == required)
+        {
+          re_.setPattern ({R"(^(?<field>[A-Ra-r]{2})([0-9]{2}((?<subsquare>[A-Xa-x]{2}){0,1})){0,1}$)"});
+        }
+      else if (Length::square == required)
+        {
+          re_.setPattern ({R"(^(?<field>[A-Ra-r]{2})[0-9]{2}(?<subsquare>[A-Xa-x]{2}){0,1}$)"});
+        }
+      else
+        {
+          re_.setPattern ({R"(^(?<field>[A-Ra-r]{2})[0-9]{2}(?<subsquare>[A-Xa-x]{2})$)"});
+        }
+      break;
+    case Length::extended:
+      if (Length::field == required)
+        {
+          re_.setPattern ({R"(^(?<field>[A-Ra-r]{2})([0-9]{2}((?<subsquare>[A-Xa-x]{2}){0,1}([0-9]{2}){0,1})){0,1}$)"});
+        }
+      else if (Length::square == required)
+        {
+          re_.setPattern ({R"(^(?<field>[A-Ra-r]{2})[0-9]{2}((?<subsquare>[A-Xa-x]{2})([0-9]{2}){0,1}){0,1}$)"});
+        }
+      else if (Length::subsquare == required)
+        {
+          re_.setPattern ({R"(^(?<field>[A-Ra-r]{2})[0-9]{2}(?<subsquare>[A-Xa-x]{2})([0-9]{2}){0,1}$)"});
+        }
+      else
+        {
+          re_.setPattern ({R"(^(?<field>[A-Ra-r]{2})[0-9]{2}(?<subsquare>[A-Xa-x]{2})[0-9]{2}$)"});
+        }
+      break;
+    }
+}
+
+auto MaidenheadLocatorValidator::validate (QString& input, int& pos) const -> State
+{
+  auto match = re_.match (input, 0, QRegularExpression::PartialPreferCompleteMatch);
+  auto field = match.captured ("field");
+  if (field.size ())
+    {
+      input.replace (match.capturedStart ("field"), match.capturedLength ("field"), field.toUpper ());
+    }
+  auto subsquare = match.captured ("subsquare");
+  if (subsquare.size ())
+    {
+      input.replace (match.capturedStart ("subsquare"), match.capturedLength ("subsquare"), subsquare.toLower ());
+    }
+  if (match.hasMatch ()) return Acceptable;
+  if (!input.size () || match.hasPartialMatch ()) return Intermediate;
+  pos = input.size ();
+  return Invalid;
+}