API nit: Use Util::AsyncOnObject everywhere instead of QTimer::singleShot

This is just to make the API easier to follow. Also cleaned up
AsyncOnObject to not default to caarse timer unless the interval is
>2000 msec.

Also we use QMetaObject::invokeMethod() directly by default if the
AsyncOnObjectCall() has when_ms <= 0 (default).

This should have no real observable change on app behavior other than
being a code cleanup & nit.

src/Util.cpp

@@ -780,7 +780,7 @@ Log::~Log()
             }
             // Fatal should signal a quit even here
             if (level == Logger::Level::Fatal && qApp) {
-                QTimer::singleShot(0, qApp, []{ qApp->quit(); });
+                Util::AsyncOnObject(qApp, []{ qApp->quit(); });
             }
         }
     }