[MapModel] Add "tooltip" concept

[mbeckem]

A tooltip on the map follows the cursor, may have an icon, and indicates what the user can do with the currently active interaction (e.g. click to select, double click to finish drawing, ...). This is repeated in mulitple packages (based on OpenLayers overlays).

Drawbacks of the current implementation:

• No enforcement that there can be only one tooltip at a time
• Code duplication
• Developer needs to work with raw html nodes / string contents (error prone)
• Developer cannot use react features inside the tooltip

Idea:

• Implement a common tooltip / overlay API in the map model that makes it easy to create "widgets" at certain map coordinates (that may also follow the mouse). Ideally: support react here (via createPortal)
• Implement an interaction API that enforces mutually exclusive cursor interactions (-> only one tooltip)
• In the future, the tooltip / overlay API could also be used to implement popups