Added geochecker button at end of description text

[Lineflyer]

To be checked whether this function is already described (was existing before) and/or it should be added or not.

[moving-bits]

In the past we had a link at the end of the cache description, but for gc geochecker only.

This has changed to a button below the description text and gets displayed if any geochecker is available (of those supported by c:geo). For checking this condition the same logic gets applied as we had in the past for the "open geochecker" menu entry. This menu entry got removed meanwhile, though, for reasons of de-duplication.

[murggel]

The link to the checker is not described in the current manual

https://manual.cgeo.org/en/cachedetails#description_tab

If it should be described, the screenshot has to be updated.