Autogen: Process files concurrently in AUTOMOC and AUTOUIC
This introduces concurrent thread processing in the `_autogen` target wich processes AUTOMOC and AUTOUIC. Source file parsing is distributed among the threads by using a job queue from which the threads pull new parse jobs. Each thread might start an independent ``moc`` or ``uic`` process. Altogether this roughly speeds up the AUTOMOC and AUTOUIC build process by the number of physical CPUs on the host system. The exact number of threads to start in the `_autogen` target is controlled by the new AUTOGEN_PARALLEL target property which is initialized by the new CMAKE_AUTOGEN_PARALLEL variable. If AUTOGEN_PARALLEL is empty or unset (which is the default) the thread count is set to the number of physical CPUs on the host system. The AUTOMOC/AUTOUIC generator and the AUTORCC generator are refactored to use a libuv loop internally. Closes #17422.
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