Feature/seletable title more like select2

[``* `#8743` Query select menu does not work exactly like select2``](https://www.openproject.org/work_packages/8743)

Wondering if it might have been easier to have just of used select2 instead of basically re-implementing it. Never!

[Note - "...to have just of..." is a tired scottish guy trying to say "...to just have..."]
I must admit that I find the implementation of ``selectNext()`` (as well as ``selectPrevious()``)  rather complex: There are a lot of loops and conditions for a task like _select next entry_. I guess it would help if we split up the method and add some additional method calls to improve readability. For instance:
```
currentIndex = getCurrentIndex();
nextIndex = getNextIndex(currentIndex);
selectIndex(nextIndex);
```
I know that this means to duplicate some loops but, otherwise, one is overwhelmed with to much implementation details.
We should also speak about variable naming: I don't have the slightest idea what a model means in this context. :disappointed: