Add grouplink css class instead of brackets

[karliwalti]

offers more flexibility to format the grouplinks.

add

.grouplink::before {
  content: "[";
}
.grouplink::after {
  content: "]";
}

to css for backwards compatibility

[kfumy]

Dear Mr. Walten, I talked to the agency who develops the extension and we found a better way, to make this more flexible and save. So we don´t merge this pull request. The reason is, that there is a change in the way, where and how the navigation group is build up.

[karliwalti]

oh, I thought I added this to the refactored code already. looking forward to the new implementation. thank you