BlHostEventFetched: Collapse duplicated/similar events to avoid over-processing

[joao-pedro-braz]

BlHostEventFetched can now collapse (i.e. skip) collapse-able events before flushing them. This avoids having to process multiple events that are all essentially identical in a single frame, which can lead to stuttering and, most likely, having the UI feel sluggish when selecting text.

[joao-pedro-braz]

Clarified BlHostEventFetcher >> fetchedEventsDo: comment and also improved how we collapse events.

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