Prevent post-compilation Qt downgrades (#2576)

* Abort the app if a Qt downgrade is detected

The app will now exit immediately if it was compiled with a Qt version
higher than the one present on the machine.

* Add function for checking the Qt version at runtime

* Re-register global D-Bus menu only if DE is Unity

src/main.cpp

@@ -46,6 +46,8 @@ Q_IMPORT_PLUGIN(QXcbIntegrationPlugin)
 
 int main(int argc, char** argv)
 {
+    QT_REQUIRE_VERSION(argc, argv, QT_VERSION_STR)
+
 #if QT_VERSION >= QT_VERSION_CHECK(5, 6, 0)
     QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
 #endif