pageIndicators: Redesign and add position-based animation

Screencast_from_22-11-19_03_20_53

Remove setCurrentPage() function, introduce setCurrentPosition() instead, which allows to have fractional positions.

Make inactive dots smaller, filled and partially transparent, as opposed to larger and fully opaque active dot. Make dots smaller overall, remove borders. Interpolate each dot between active and inactive state based on scroll position.

Make it impossible to "uncheck" the active dot.

Thanks Florian Müllner for parts of the code.

Fixes #1932 (closed)