Collapsible optimisation, first tests
Remove legacy and deprecated components
Remove legacy styles
Use 2.3 Duplicator API
Don't manipulate the DOM in order to lock ordering. This speeds up rendering other animations like collapsing.
Move Duplicator animations to CSS
Move Collapsible animations to CSS
Clean up section editor interactions
Simplify backend frames
Additional fixes related to backend frames
Remove .collapsed class after expanding all items
Move styles to proper context (duplicator to collapsible)
Delay removal of `.js-animate` to make sure the transition actually ended
Fix drawer sizing and window scroll behaviour

Fixes #1876 and closes #1883
Remove gradients from table selection in order to keeps styles consistent (see Duplicators)
Add `.instance` classe to collapsible items for compatibility reasons.