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),