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.

widgets/SplashScreen.cpp

@@ -0,0 +1,48 @@
+#include "SplashScreen.hpp"
+
+#include <QPixmap>
+#include <QVBoxLayout>
+#include <QCheckBox>
+#include <QCoreApplication>
+
+#include "revision_utils.hpp"
+#include "pimpl_impl.hpp"
+
+#include "moc_SplashScreen.cpp"
+
+class SplashScreen::impl
+{
+public:
+  impl ()
+    : checkbox_ {"Do not show this again"}
+  {
+    main_layout_.addStretch ();
+    main_layout_.addWidget (&checkbox_, 0, Qt::AlignRight);
+  }
+
+  QVBoxLayout main_layout_;
+  QCheckBox checkbox_;
+};
+
+SplashScreen::SplashScreen ()
+  : QSplashScreen {QPixmap {":/splash.png"}, Qt::WindowStaysOnTopHint}
+{
+  setLayout (&m_->main_layout_);
+  showMessage ("<h2>" + QString {"WSJT-X v" +
+        QCoreApplication::applicationVersion() + " " +
+        revision ()}.simplified () + "</h2>"
+    "V2.0 has many new features.<br /><br />"
+    "The release notes have more details.<br /><br />"
+    "Send issue reports to wsjtgroup@yahoogroups.com, and be sure to save .wav<br />"
+    "files where appropriate.<br /><br />"
+    "<b>Open the Help menu and select Release Notes for more details.</b><br />"
+    "<img src=\":/icon_128x128.png\" />"
+    "<img src=\":/gpl-v3-logo.svg\" height=\"80\" />", Qt::AlignCenter);
+  connect (&m_->checkbox_, &QCheckBox::stateChanged, [this] (int s) {
+      if (Qt::Checked == s) Q_EMIT disabled ();
+    });
+}
+
+SplashScreen::~SplashScreen ()
+{
+}