[PATCH v2 0/1] MR5861: mshtml: Implement visibilitychange event.

[Gabriel Ivăncescu (＠insn)]

Since `EVENT_SYSTEM_MINIMIZESTART` and `EVENT_SYSTEM_MINIMIZEEND` are implemented now (thanks to Esme), this is implementing visibility change events using them. We have to keep track of the browsers to iterate all the documents from the hook.

Note that I tried to refcount the per-thread hook and add it only when necessary (on a visiblity change listener), since it can slow down the entire prefix.

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  v2: mshtml: Implement visibilitychange event.

https://gitlab.winehq.org/wine/wine/-/merge_requests/5861