fix: check if overlay was already instantiated before creating a new one

[silvester-pari]

This PR goes in a different direction than #555: when the select interaction is initialized, first check if an eox-map-tooltip was already instantiated on the map, if so, reuse that overlay, if not, create a new one.
This should fix the issue with not finding the DOM element, but means only one tooltip can exist per map.

[RobertOrthofer]

While this does still seem a little hacky to me, i like the approach of going back to having a single tooltip element for the entire map.
This disregards tooltips for click selection, which is ok I suppose for the current use cases and the current API.

[silvester-pari]

While this does still seem a little hacky to me, i like the approach of going back to having a single tooltip element for the entire map. This disregards tooltips for click selection, which is ok I suppose for the current use cases and the current API.

Thanks for your review. While this indeed "breaks" the functionality to have e.g. different tooltips on hover and on click, the basic tooltip functionality should be more robust without introducing breaking changes to the API.