tildify.el: introduce a `tildify-foreach-region-function' variable

tildify.el: introduce a `tildify-foreach-region-function' variable

* textmodes/tildify.el (tildify-foreach-region-function): New
variable specifying a function determining portions of buffer that
should be tildified.  It allows major modes to create a filtering
function more elaborate than a set of regular expressions.
Initialised to `tildify--deprecated-ignore-evironments' by default
to handle now deprecated `tildify-ignored-environments-alist'
variable.
(tildify--foreach-region): A new function that takes
`tildify-foreach-region-function' into account and calls callback
for regions of the buffer that should be tildified.
(tildify-foreach-ignore-environments): A new function which can be
partially applied and used as `tildify-foreach-region-function'.
(tildify-ignored-environments-alist, tildify--pick-alist-entry):
Mark as obsolete.
(tildify--find-env): Rename from `tildify-find-env' and mark as
obsolete.
(tildify--deprecated-ignore-evironments): New function,
immediately marked as obsolete, used to handle deprecated
`tildify-ignored-environments-alist'.

* textmodes/tex-mode.el (tex-common-initialization): Set
`tildify-foreach-region-function' variable in all variants of TeX
mode since `tildify-ignored-environments-alist' variable is now
empty by default.

* nxml/nxml-mode.el (nxml-mode): Ditto in `nxml-mode'.

* textmodes/sgml-mode.el (sgml-mode): Ditto in `sgml-mode'.