Now layout files and modules can extend the cite engines or completely
overwrite them, and modify the cite formats.
Any CiteEngine definition in a layout/module will completely overwrite
those by cite engine files.
AddToCiteEngine will extend them (add if they do not exist yet).
Any CiteFormat definition in a layout will be preferred to those in cite
engines. CiteFormat definitions that are not touched by the former are
still active, though (so, as opposed to CiteEngine, a CiteFormat does
not completely overwrite those by the engine files).
Layout format change.
The current spelling is not strictly wrong, but flagged as unusual or
historical by some authorities. It is also found fault with many
spell checkers. Thus we decided to move to the more standard "-ible"
form once and for all.
See #10678 for discussion
This last part updates the layout format and changes collapsable color.
This will all also all be backported to 2.3.x, for the sake of backwards
compatibility (cherry-picking).
