unify and fix position left and right handling

Afaik position `left` and `right` should do the same thing with the only difference is whether the tooltip is left or right of the element. Therefore I tried to unify the handling.

Besides this, there was a bug in position `right` if the element would have fallen below the screen because of a missing `else`.

I am **not** sure about those lines and why they are there:

```
if (this._options.showStepNumbers == true) {
  tooltipLayer.style.top = '15px';
}
```

They were only at position `left` before. I don't know why the step numbers should affect the top positioning of the tooltip, but maybe this has a purpose so I left it in.