Flymake uses some new fringe bitmaps

Flymake uses some new fringe bitmaps

Also fix behaviour whereby flymake wouldn't react to a change in the
variable.

* lisp/progmodes/flymake-ui.el (flymake-error-bitmap)
(flymake-warning-bitmap): Update bitmaps.
(flymake-note-bitmap): New defcustom.
(flymake-double-exclamation-mark): New bitmap.
(flymake-error, flymake-warning, flymake-note)
(flymake--highlight-line): 'bitmap property must be a symbol.
Also set default face to flymake-error.
(flymake--fringe-overlay-spec): Bitmap property can be a
variable symbol.