tildify.el: introduce a `tildify-space-string' variable

tildify.el: introduce a `tildify-space-string' variable

* textmodes/tildify.el (tildify-space-string): New variable for
specifying representation of a hard space -- a no-break space by
default.  Being a buffer-local variable it is much easier to
handle than `tildify-string-alist' that has been used so far.  It
also works better with derived modes.
(tildify-string-alist): Mark as obsolete.

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

* nxml/nxml-mode.el (nxml-mode): Ditto in `nxml-mode'.  If
encoding supports it use no-break space instead of character
entity; this changes previous default which used a numeric
reference.

* textmodes/sgml-mode.el (sgml-mode): ditto in `sgml-mode'.  If
encoding does not support no-break space, use numeric reference;
this changes previous default which used named entity (“ ”)
in HTML mode.