Refactor Map component into generic superclass

Refactor the workflow specific Demap into a generalized superclass that will be extended within the scope of each workflow.

Provide API to support:

• provide possibility to set view projection (default: 4326)
• add arbitrary base layer (in addition to OSM) (with mandatory tile!)
• add arbitrary overlay layer (title, style, geometry, click_callback)
• show all layers in a layer overview panel (provide possibility to select layer as region of interest, see #18 (closed))