(Cheap) Support for multiple segmentation layers

[philippotto]

It would already be useful to be able to have two segmentation layers in a datasource-properties.json and then have only one rendered in webKnossos.

The different degrees of this feature could be:

• simply pick first segmentation layer and ignore/drop the other one
• let the user pick a segmentation layer before the page loads fully (via URL parameter for example)
• let the user switch between segmentation layers in UI (could simply reload the entire page while setting the URL parameter)
• let the user switch between the layer without reloading (use reloadBucket functionality?)

Also: A lot of code assumes, that there is only one segmentation layer. That code could stay mostly intact by changing getSegmentationLayer to getActiveSegmentationLayer (the user would choose which seglayer is the active one).

As a very first step for this issue, verify that a segmentation layer does not need to be named "segmentation"