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.
Add a new class QgsDockWidget which has finer control over setting
and retrieving the dock visibility, to account for dock widgets
which are open but hidden by other docks
