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.
QgsNumericSortTreeWidgetItem had a giant TODO saying "make it work".
This makes it work, and adds some other useful features like
being able to specify custom sort value and force items to always
sort on top.
