refactor add_layer and remove_layer

[12rambau]

In the latest release of ipyleaflet, all components (controls, layers etc...) are added to the map using add instead of add_layer and add_control.

In the lib we are overwriting the add_layer in multiple ways. The implementation does not rise DeprecationWarning anymore but we should consider refactoring the way we add layers to the map so that newcomers don't get lost.