Cleanup: Implicitly free QApplication

Instead of deleting the Application explicitly in exit_ui(),
use a unique_ptr to have the instance automatically freed.
This is the pattern given in the Qt-docs:
https://doc.qt.io/qt-5/qapplication.html

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

subsurface-desktop-main.cpp

@@ -43,7 +43,7 @@ int main(int argc, char **argv)
 	int i;
 	bool no_filenames = true;
 	QLoggingCategory::setFilterRules(QStringLiteral("qt.bluetooth* = true"));
-	new QApplication(argc, argv);
+	std::unique_ptr<QApplication> app(new QApplication(argc, argv));
 	QStringList files;
 	QStringList importedFiles;
 	QStringList arguments = QCoreApplication::arguments();