[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