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.
The file qgsexpressions.h has grown to one monolithic piece over the
years. This makes it hard to maintain and slows down compilation because
even small changes at one end will result in recompiling big parts of
the source tree. It also requires the compiler to keep track of all
these implementation details for said big parts of the source tree.
This splits this implementation into smaller pieces. There are soe API
changes connected to this, but since these can be considered
implementation details, on which not many plugins rely, this shouldn't
have a big impact on the ecosystem outside the source tree.
