sp-delete-char and friends do not respect strict mode with delete-selection-mode on

[Fuco1]

Expected behavior

If the strict mode is on and delete-selection-mode is enabled, we should not allow to delete regions that would break structure.

Actual behavior

Strict mode checks are ignored and it is possible to delete region leading to breakage.

Steps to reproduce the problem

Enable the two modes, then select text and hit backspace.

Solution

(put 'sp-backward-delete-char 'delete-selection 'supersede)
(put 'sp-delete-char 'delete-selection 'supersede)

should use a callback instead of supersede, and return nil if we can't delete the region, and return supersede when we can.