(gdb-source-window): New variable.

(gdb-source-window): New variable.
Re-introduce the concept of a source window.
(gdb-get-buffer-create): Rename from gdb-get-create-buffer for
consistency with get-buffer-create.
(def-gdb-auto-update-handler, gdb-info-locals-handler)
(gdb-data-list-register-values-handler)
(gdb-stack-list-locals-handler): Try to preserve window-start as
well as window-point.
(gdb-display-source-buffer): New function (old concept).
(gdb-goto-breakpoint): Use it.