Collapsable -> Collapsible (part 1)

Collapsable -> Collapsible (part 1)

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 only covers the usage in comments and the like. More to follow.

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