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.
This means if a plugin registers a custom drop handler, but crashes
before it can deregister it, at least it won't leave the main
app in a crashy state.
and dropped file names
This allows QgsCustomDropHandlers which aren't directly associated
with providers (e.g. for plugins to handle dropped files of
a certain type)
