Change layout and structure of the startup page

Since we started the development with the similarity workflow, the index page is showing the elements for exactly this workflow. In order to select the specific workflow before anything is loaded, we have to restructure our landing page.

As a first version I suggest we create a small tree structure in the navigation drawer on the left-hand side, e.g.:

Showcase A: Methane
├── Workflow 1: Differences
└── Workflow 2: ...

Showcase B: Flood
├── Workflow 1: Similarity
└── Workflow 2: Change
.
.
.

Therefore we need to refactor the current index page into the landing page for the Similarity workflow, which is pulled up, when the user clicks on the corresponding entry in the navigation drawer tree and design a new startup landing page, which could be empty for now.

Furthermore we have to provide some kind of workflow template, so its easier to add new ones and the look and feel of each workflow is somewhat similar.