Rename GTK client files to match the classes they contain (#2073)

This also brings naming closer to Qt client which simplifies things a
bit.

gtk/PrefsDialog.h

@@ -0,0 +1,36 @@
+/*
+ * This file Copyright (C) 2007-2021 Mnemosyne LLC
+ *
+ * It may be used under the GNU GPL versions 2 or 3
+ * or any future license endorsed by Mnemosyne LLC.
+ *
+ */
+
+#pragma once
+
+#include <memory>
+
+#include <gtkmm.h>
+
+class Session;
+
+class PrefsDialog : public Gtk::Dialog
+{
+public:
+    ~PrefsDialog() override;
+
+    static std::unique_ptr<PrefsDialog> create(Gtk::Window& parent, Glib::RefPtr<Session> const& core);
+
+protected:
+    PrefsDialog(Gtk::Window& parent, Glib::RefPtr<Session> const& core);
+
+private:
+    class Impl;
+    std::unique_ptr<Impl> const impl_;
+};
+
+enum
+{
+    MAIN_WINDOW_REFRESH_INTERVAL_SECONDS = 2,
+    SECONDARY_WINDOW_REFRESH_INTERVAL_SECONDS = 2
+};