Fix face naming

Make faces adhere to the Emacs conventions, that is, remove the trailing
`-face` from their names.  Mark the old faces `flycheck-warning-face`
and `flycheck-error-face` obsolete.
Make fringe faces customizable independently
Greatly improve the default faces

Use underlines instead of foreground faces if possible, to resemble
highlighting of modern IDEs
Merge branch 'improve-faces'

* improve-faces:
  Greatly improve the default faces
  Make fringe faces customizable independently
  Fix face naming