Fix most of critical issues reported by Sonar (GTK client) (#2309)

* (C++) Macros should not be used to define constants

* (C++) Memory should not be managed manually

* (C++) "void*" should not be used in typedefs, member variables, function parameters or return type

* (C++) When the "Rule-of-Zero" is not applicable, the "Rule-of-Five" should be followed

* (C++) "switch" statements should have "default" clauses

* (C++) "explicit" should be used on single-parameter constructors and conversiosn operators

* (C++) Non-const global variables should not be used

gtk/StatsDialog.h

@@ -12,6 +12,8 @@
 
 #include <gtkmm.h>
 
+#include <libtransmission/tr-macros.h>
+
 class Session;
 
 class StatsDialog : public Gtk::Dialog
@@ -19,6 +21,8 @@ class StatsDialog : public Gtk::Dialog
 public:
     ~StatsDialog() override;
 
+    TR_DISABLE_COPY_MOVE(StatsDialog)
+
     static std::unique_ptr<StatsDialog> create(Gtk::Window& parent, Glib::RefPtr<Session> const& core);
 
 protected: