pageIndicators: Redesign and add position-based animation

pageIndicators: Redesign and add position-based animation

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 https://gitlab.gnome.org/GNOME/gnome-shell/issues/1932

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/843