Enhanced sticky headers to be more versatile, fixed bugs with clones

- Fixed bugs with headers having improperly computed positions when using same-depth headers (headers outside viewport were being included in the computations)
- Fixed an issue where headers taller than the "scrollaway margin" were disappearing early instead of scrolling away slowly
- Updated support for filter/rich header sections
- Updated headers to use the original header elements as the sticky ones (now using the clones as placeholders) -- This fixes input issues with headers that contain rich content like a filter section
- Fixed bugs with clones being cleaned up/accessed improperly

These commits were cherry-picked from the pbc-filters branch so that they may be merged earlier.