Obsolete Flymake's flymake-diagnostic-types-alist

Obsolete Flymake's flymake-diagnostic-types-alist

That varaiable was an association between symbols and properties,
effecively duplicating symbol's property lists.  It is simpler to just
put properties on symbols.  Backward compatibility to the old variable
has been kept.

* doc/misc/flymake.texi (Flymake error types): Don't mention
flymake-diagnostic-types-alist.
(Flymake error types): Rework section.
(Backend functions): Refill a paragraph.
(Flymake utility functions): Don't mention
flymake-diagnostic-types-alist.
(Proc customization variables): Don't mention
flymake-diagnostic-types-alist.

* etc/NEWS: Mention obsoletion of flymake-diagnostic-types-alist.

* lisp/progmodes/flymake-proc.el
(flymake-proc--diagnostics-for-pattern): Don't use
flymake-diagnostic-types-alist.

* lisp/progmodes/flymake.el: Rewrite commentary.
(flymake-make-diagnostic, flymake-mode, flymake-goto-next-error)
(flymake-goto-prev-error): Don't mention
flymake-diagnostic-types-alist in docstring.
(flymake-diagnostic-types-alist): Make obsolete.
(:error, :warning, :note): Put flymake-category in these symbols.
(flymake-error, flymake-warning, flymake-note): Put
`flymake-bitmap', not `bitmap' in these symbols.
(flymake--lookup-type-property, flymake--highlight-line): Rewrite.
Honor flymake-diagnostic-types-alist for backward
compatibility.

* lisp/progmodes/python.el (python-flymake-msg-alist): Don't
mention flymake-diagnostic-types-alist.