Most frames there will be no change in project settings, and it makes no sense to read settings every frame in case of changes, as a large number of string compares are involved.
This PR adds a signal to ProjectSettings that can be subscribed to in order to keep local settings up to date with ProjectSettings.
In addition a function `ProjectSettings::has_changes()` is provided for objects outside the signal system (e.g. Rasterizers).
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