add variant type aliases, as preparation to migrate away from QVariant

[jkarneges]

As part of our ongoing process of migrating away from Qt types, this PR is the first in a series for addressing QVariant.

The plan is to do something similar to what we're doing with the Cow types, by introducing a replacement type, Variant, that behaves exactly the same way as QVariant and is initially a wrapper around it. Later, the implementation can be substituted with something else, likely std::variant. Unlike the Cow types, however, which are being slowly rolled out throughout the project, for Variant the plan will be to roll it out to the whole project in one shot via a type alias. The reason for doing it this way is to avoid deep copy conversions between our Variant and QVariant, which are hard to avoid when the types are distinct and variants are nested within variants.

Steps:

1. Add Variant* type aliases (this PR). Easy change, nothing using it yet.
2. Change all mentions of QVariant* in the whole project to Variant*. The actual types would still be QVariant* so this should be completely safe with no worry about behavioral changes.
3. Replace Variant* type aliases with Variant* classes that wrap QVariant*, forwarding to an inner instance. No change to the call sites.
4. Eventually, change the implementation of the Variant* classes with something not Qt-based.