UI: Split out Whats New dialog, fix crash on shutdown

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CEF crashes on shutdown if any browser window remains open during the
shutdown flow. Reproducible 100% of the time on Linux with What's New.
The previous implementation of the What's New dialog correctly deleted
all the Qt widgets, but the CEF internal event flow would only call the
"preparing to close" function, and never the "window safely closed"
function. Moving the code (practically unchanged) to a custom QDialog
solves the problem entirely, and is more consistent with other uses.

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500 Internal Server Error

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