Add EventHandler to layer object

[hansthen]

In combination with JsCode this makes it easier for users to add on method calls for event handling without extending Folium itself.

The functionality was inspired by PR #1866 by @yschopfer19. The PR was not accepted yet, because of concerns with code duplication. In the approach taken in the current PR, #1866 would not be necessary anymore, as the requested changes could be added completely in client code space.