Various updates in preparation for GsPluginJobUpdateApps

I’m hoping to land the API changes for the next bit of the threading rework, GsPluginJobUpdateApps, in time for the API freeze. Sorry it’s so close to the wire.

Here are some preparatory commits which come before the main branch. Some of them change API, but are less critical and should be fairly straightforward to review.

Signed-off-by: Philip Withnall

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