Make smooth scroll animations respect global animation speed
This MR makes the smooth scroll animations respect the global animation scale factor, while not changing the current speeds at all for people who are using the default settings. This fixes https://bugs.kde.org/show_bug.cgi?id=420755.
There are three commits, which should not be squashed. One defines new central variables for the animation durations, another other scales them according to the global animation speed value, and a third uses KConfigWatcher to respond to changes to the config value while Okular is running.