This implements some events related to document/window load and unload / visibility changes. The latter is related in that native partially implements it, and only sends it when it is minimized or restored from being minimized, not on navigation or anything else.
The main issue for me is the visibilitychange patch. Since we don't control the container hwnd, and it can even be in another process, there is no way to be notified cleanly when it changes its minimization state. We can't subclass it either, as the tests show, it's not subclassed.
I implemented it now using an internal timer that polls for that, since it was the least invasive and works in all cases, including other process hwnd. Hooks wouldn't be able to do that, and are also more invasive IMO. I'm hoping for better ideas, though.
-- v5: mshtml: Implement pagehide event. mshtml: Implement unload event. mshtml: Dispatch beforeunload event to the window. mshtml: Implement pageshow event. mshtml: Store the nsevent listener descs in a table. mshtml: Add visibilitychange event stub. mshtml: Dispatch Gecko events to the window if there's no node. mshtml: Do not register Gecko load event handler twice. mshtml: Allow EVENT_BIND_TO_TARGET to bind to the window.