desktop: make MainWindow a "global object"

Thus, we don't have to delete it explicitly on exit.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/mainwindow.cpp

@@ -111,7 +111,7 @@ extern "C" void showErrorFromC(char *buf)
 	emit MainWindow::instance()->showError(error);
 }
 
-MainWindow::MainWindow() : QMainWindow(),
+MainWindow::MainWindow() :
 	appState((ApplicationState)-1), // Invalid state
 	actionNextDive(nullptr),
 	actionPreviousDive(nullptr),

subsurface-helper.cpp

@@ -58,16 +58,13 @@ void init_ui()
 #ifndef SUBSURFACE_MOBILE
 	register_qml_types(NULL);
 
-	MainWindow *window = new MainWindow();
+	MainWindow *window = make_global<MainWindow>();
 	window->setTitle();
 #endif // SUBSURFACE_MOBILE
 }
 
 void exit_ui()
 {
-#ifndef SUBSURFACE_MOBILE
-	delete MainWindow::instance();
-#endif // SUBSURFACE_MOBILE
 	free_globals();
 	free((void *)existing_filename);
 }