visfilt-choose: set buffer read-only
visfilt-choose: more detailed error messages
vf-buffer-list: add more keys to keymap
whitespace cleanup
visfilt-filter-list: convert to use cl-loop

Convert to use cl-lib instead of cl
visfilt: use lexical binding
visfilt-filter-functions: replace type-of with predicates

Remove alist of with types as keys with a list of predicates.

- Rename from visfilt-filter-function-alist.
- Add recursive function visfilt--get-filter to parse the list.
visfilt-regenerate-keymap: use DEL for terminal

When running emacs on a terminal, the backspace key is not
available. Use DEL, which is more proper also.