Flymake: futher enhance echo-area appearance of diagnostics

Flymake: futher enhance echo-area appearance of diagnostics

Also describe new 'echo-face' property in the Flymake manual, and fix
it's mistaken mention of a non-existing 'flymake-severity' property.

* doc/misc/flymake.texi:
(Flymake error types): Describe new properties and correct mention
of 'severity' property.

* lisp/progmodes/flymake.el:
(flymake-diagnostic-oneliner): Rework.
(flymake-error, flymake-warning, flymake-note): Add new 'echo-face' property.
(flymake--highlight-line)
(flymake-eldoc-function)
(flymake--tabulated-entries-1): Use flymake-diagnostic-oneliner
(Version): Bump to 1.3.3