Qt uses a signed value to represent indices. We should follow this
convention where applicable to avoid unnecessary sign-conversion
warnings, as well as making it easier to interoperate with other aspects
of Qt.
While we're at it, we can also make a sign-conversion explicit.
Since C++17, the introduction of deduction guides for locking facilities
means that we no longer need to hardcode the mutex type into the locks
themselves, making it easier to switch mutex types, should it ever be
necessary in the future.
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