Improve UI performance

This pull request improves the responsiveness of the backend UI. It replaces most jQuery based animations with CSS transitions, simplifies the `.frame` definitions and fixes the following issues:

- #1885: Expand/collapse animation in section editor is teribly wonky
- #1883: Apply fixed height only if drawer is available.
- #1876: Back button always takes you to the top of a backend page
- #1884: Fixing the toggle controls on backend pages makes the browser vomit

In order to keep transitions smooth the following backend behaviours changed:

- The `expand all` and `collapse all` button in the section editor won't be pinned on top of the screen when scrolling anymore. Fixing the legend resulted in continuous full page repaints on scroll and when animating the field instances, making animations look wonky. 
- The scripts taking care of collapsing, ordering and duplicating no longer add state classes when animating because this proofed to be too time expensive. These classes where available for about 0.25s, e. g. we added `.collapsing` before starting an animation and removed it again right after. While this is a small change, it's **breaking our JavaScript and CSS API**. Sadly, it's not an optional change because it's needed for the overall speed improvements and so this pull request should _not be added in a maintenance release_.
- Duplicators no longer support the old 2.2 API (in 2.2 Duplicator have been applied to ordered lists, since 2.3 they are applied to a wrapping `div.frame`), all legacy styles and interactions have been removed.
- There are a few small visual changes – like removed box shadows on frames – to reduce the number of repaints.

Due to the CSS simplifications in our `.frame` definitions, it's possible that the styling of extensions break. These issues should be easy to solve (I already notices that Date and Time needs some love). An other reason why this should not be added in the maintenance release.

If you notice general styling issues related to these changes, please post here.