Create app.timer for timing UI-independent functions

[falkoschindler]

As discussed in #3187, ui.timer works like a UI element: It starts running when a page is created and stops when removing the timer or its parent container. This is handy in some cases, but isn't very helpful in others.

Therefore we propose an app.timer function that behaves similar to ui.timer, but runs on an app level, independent of connected clients. It will, however, have no UI context to work with. So it is primarily meant to run non-UI code.