[QtGui] Replace QThreadStorage<T> with thread_local T or thread_local optional<T>

See epic for more information. Use thread_local T if the object construction is constinit (then add Q_CONSTINIT in front, too), otherwise use thread_local std::optional<T> (and also add Q_CONSTINIT).

Acceptance criteria:

• All uses of QThreadStorage are removed from the module.
• QT_NO_QTHREADSTORAGE is defined in .cmake.conf (cf. tasks in https://codereview.qt-project.org/q/topic:Q_FOREACH).