use Variant alias types everywhere in place of QVariant

[jkarneges]

This follows up on #48319 by updating the whole codebase to use the aliases. This PR includes the following changes:

• Use Variant everywhere instead of QVariant.
• Use VariantHash everywhere instead of QVariantHash.
• Use VariantMap everywhere instead of QVariantMap.
• Use VariantList everywhere instead of QVariantList.
• Replace all #include <QVariant*> directives with #include "variant.h".
• Add an include guard to qtcompat.h.

Since the types are aliases to the Qt types, the code still actually uses the Qt types and compiles to the same thing, so there should be no change in behavior.

Note that even with these changes, call sites are not fully Qt-free. For example, the QHashIterator type is used to iterate VariantHash, and variant types are compared against enum values like QMetaType::QVariantHash. These will be addressed later.