GTK: delete all gtkmm objects before destrying Gtk::Main.

Otherwise this results in harmless but annoying spam on applciation
shutdown.

See https://gitlab.gnome.org/GNOME/gtkmm/issues/22.

src/platform/entrygui.cpp

@@ -29,8 +29,8 @@ int main(int argc, char** argv) {
 
     Platform::Close3DConnexion();
     SS.Clear();
-
     SK.Clear();
+    Platform::ClearGui();
 
     return 0;
 }

src/platform/gui.h

@@ -380,6 +380,7 @@ void OpenInBrowser(const std::string &url);
 void InitGui(int argc, char **argv);
 void RunGui();
 void ExitGui();
+void ClearGui();
 
 }
 

src/platform/guigtk.cpp

@@ -1475,6 +1475,9 @@ void RunGui() {
 
 void ExitGui() {
     gtkMain->quit();
+}
+
+void ClearGui() {
     delete gtkMain;
 }
 

src/platform/guimac.mm

@@ -1479,5 +1479,7 @@ void ExitGui() {
     [NSApp terminate:nil];
 }
 
+void ClearGui() {}
+
 }
 }

src/platform/guinone.cpp

@@ -154,6 +154,8 @@ void ExitGui() {
     exit(0);
 }
 
+void ClearGui() {}
+
 }
 
 }

src/platform/guiwin.cpp

@@ -1677,5 +1677,7 @@ void ExitGui() {
     PostQuitMessage(0);
 }
 
+void ClearGui() {}
+
 }
 }