Run clang-tidy modernize-use-override to remove all the redundant
virtual keywords from overridden methods, and add some missing
overrides.
Another benefit is that this has also added the overrides
on destructors, which will cause a build failure if a base
class is missing a virtual destructor.
- map layers have associated QgsMapLayerLegend instance
- QgsMapLayerLegend is interface for generation of legend with some default implementations
- QgsLayerTreeModelLegendNode is interface for representation of legend item in layer tree model with some default implementations
