(breakpoint-enabled-icon, breakpoint-disabled-icon):

(breakpoint-enabled-icon, breakpoint-disabled-icon):
Initialize margin area images to nil.
(breakpoint-bitmap): New defvar for breakpoint fringe bitmaps.
(breakpoint-enabled-bitmap-face)
(breakpoint-disabled-bitmap-face): New faces for bpt in fringe.
(gdb-info-breakpoints-custom): Use gdb-remove-breakpoint-icons.
(gdb-info-breakpoints-custom): Use gdb-put-breakpoint-icon.
(gdb-mouse-toggle-breakpoint): Handle bpt in fringe.
(gdb-reset): Use gdb-remove-breakpoint-icons.
(gdb-put-string): Add dprop arg to specify alternative display
property (for setting fringe bitmap).
(gdb-remove-strings): Doc fix.
(gdb-put-breakpoint-icon): New defun which displays a breakpoint
icon in fringe (if available), or else as icon or text in display
margin.  Creates necessary icons in breakpoint-bitmap,
breakpoint-enabled-icon, and/or breakpoint-disabled-icon.  Also
make left window margin if required.
(gdb-remove-breakpoint-icons): New defun to remove breakpoint
icons inserted by gdb-put-breakpoint-icon.  Remove left margin if
no longer needed.
(gdb-assembler-custom): Use gdb-remove-breakpoint-icons and
gdb-put-breakpoint-icon.
(gdb-assembler-mode): Don't set left-margin-width here.