Add kinetic / inertial scrolling (mouse and touchscreen) + smooth navigation with cursor keys
I added kinetic / inertial scrolling to ease the use of Okular on touchscreen devices, using Qt's QScroller. I think it's much nicer even for mouse usage, it may seem to scroll "too much", but, after reducing the length of the scroll, I saw that using the default (as Qt docs suggests) gives the best impression, and it's easier to navigate long documents.
I left the "mouse cursor wraps when going at the edge of the screen" mechanism intact, but if you want it can be removed since now it's way less painful to scroll a long document.
Also, I made it so that there is a nice animation when using the up / down keys to navigate the document, instead of having discrete steps.
Overall there are no (voluntary) regressions and I tested everything to be reasonably bug-free.