remove layer from layergroup

[Kilometerfresserin]

What is the feature?

• new method remove_layer_from_layergroup: works analogously to add_layer_to_layergroup and can be used to remove a single layer from an existing layergroup
• refactoring of add_layer_to_layergroup to avoid code duplication between add_layer_to_layergroup and remove_layer_from_layergroup (separate method for xml construction is shared between the two methods)

How to test?

I've added some new unittests for the new method to the existing unittest class; they can be run by executing python -m unittest tests.test_layergroup

Closes #105

[iamtekson]

I will review it soon! give me this week. Thanks.