Instead of processing all `rcc` invocation requests in the
_autogen target that calls `cmake -E cmake_autogen ...` once,
use a dedicated custom command that calls
`cmake -E cmake_autorcc ...` for each `.qrc` file.
This allows parallel `.qrc` file processing and reduces the
workload (and complexity) in the _autogen target.
If only `AUTORCC` is enabled, the _autogen target won't be created
at all since it is now used for `AUTOMOC` and `AUTOUIC` only.

For `.qrc` files that are GENERATED a custom target is used
instead of a custom command.

Closes #17161