[TOC] Support multiple (arbitrary) grouped / nested layers

[bierett]

As a user I want to support multiple (arbitrary) grouped / nested layers.

Acceptance criteria:

• after clicking on a button, the layers visibility in the map are change to not visible (current behavoir in toc-package)
• the state of the controls are synchronized with the actual visibility state of the each layer in the map (current behavoir in toc-package)
• the controls visually represents the current state of the layers visibility (current behavoir in toc-package)
• the toc is generated with multiple grouped / nested layers
• every group can contain a custom title
• show / hide all layers in a group

[mbeckem]

• GroupLayer support has been implemented in PR Implement support for GroupLayers (#222) #371, this allows arbitrary nesting (with custom titles and other layer attributes)
• "Hide All/Show All` does not exist yet (on the group layer level). There is only a top level "Hide All Layers" action.
• Should "Hide / Show all" collapse the children (i.e. hide them in the toc) or should it toggle the visibility on the map? Toggling the visibility on the parent group layer already hides / shows the children as well, so this requirement might already be fulfilled.

[bierett]

"Hide all/Show all` on the group layer level isn't needed.
“Hide all” on top level layer is sufficient.