Add option to disable smooth scrolling

Even though the animated smooth scrolling transitions are now affected
by the global animation speed setting in Plasma, this is not totally
satisfactory for the full pool of people who hate smooth scrolling, as
it does not address the cases of people using Okular outside of Plasma
or people who generally want animations but just not in Okular's
scrolling implementation.

Accordingly, there is now a GUI option to disable smooth scrolling in
Okular's settings window.

BUG: 420755
FIXED-IN: 20.12