Collapsable -> Collapsible (part 2)

Collapsable -> Collapsible (part 2)

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 part covers the most tricky part: the internal naming.
Translations and layouts will follow.

This will all also all be backported to 2.3.x, for the sake of backwards
compatibility (cherry-picking).