Added smooth scroll setting to the workspace options kcm

Added an option to enable/disable smooth scrolling to the workspace options kcm. Most software that implements smooth scrolling itself also has an option to enable/disable it based on the user's preference, so I think it's a good idea for KDE to have this option as well. I previously discussed this with some of the Kirigami developers (as most of the KDE software that has smooth scrolling is built with Kirigami) and they said they'd prefer to make it a system-wide setting rather than something like a Kirigami-specific environment variable.