Improve asynchronous operations / Use observer pattern

[Avasam]

Don't compare images in the main thread.
Leverage callback API with timespan for capture methods that have 'em, rather than constantly poking at set interval.

Consider multithreading for CPU-intensive parallel operations.
Consider moving threads to asyncio to reduce potential cross-threads issues that forces us to use signals.
Actually make better use of Qt's slot/signals system and/or move to an observable system to share events like "new frame ready".

Add global error-catching for async code (right now exceptions raised in threads are lost unless we explicitly catch them)

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Moved from Avasam#36

[Avasam]

I'll note here that Desktop Duplication (at least using D3DShot) doesn't get a new image unless the visuals actually changed on screen (ie: limited by target window and monitor's refresh rate). Causing a very low score on the Max FPS test for static captures.