Collapsable -> Collapsible (part 4)

Collapsable -> Collapsible (part 4)

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).

(cherry picked from commit 4345331870cb3207dde3930a932c77bebe81dbda)