app: in gimp-parallel, cancel ongoing async operations upon exit
When shutting-down gimp-parallel, cancel and/or abort any ongoing and queued async operations, instead of finishing them (async operations that already started executing will be canceled, but execution will be blocked until they're finished.) This is especially important since we're shutting down gimp-parallel before the destruction of data factories. This commit causes any ongoing async operations of the factories to be canceled on shutdown, rather than waiting for them to finish normally.
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