ibuffer-saved-filters: Remove extra nesting level

ibuffer-saved-filters: Remove extra nesting level

Fix Bug#25049.
* lisp/ibuf-ext.el (ibuffer-saved-filters): Remove extra
nesting level; add transparent setter to adjust old-format values;
update doc string.
(ibuffer-save-filters): Remove extra level of nesting
in ibuffer-saved-filters values when saving new filters.
(ibuffer-old-saved-filters-warning): New variable with
clickable message with repair options to be displayed
as a warning if 'ibuffer-repair-saved-filters' detects
a format mismatch.
(ibuffer-repair-saved-filters): Add new command to check and
repair saved filters format.
(ibuffer-included-in-filter-p, ibuffer-decompose-filter):
Change access of saved filter data (cadr->cdr) to account
for reduced nesting.
* test/lisp/ibuffer-tests.el (ibuffer-save-filters):
New test; check that filters are saved in the proper format.