(gdb): New group.

(gdb): New group.
(gdb-debug-log-max, gdb-enable-debug)
(gdb-cpp-define-alist-program, gdb-cpp-define-alist-flags)
(gdb-show-main, gdb-many-windows, gdb-use-separate-io-buffer)
(gdb-speedbar-auto-raise, gdb-use-colon-colon-notation)
(gdb-show-changed-values, gdb-max-children)
(gdb-delete-out-of-scope, gdb-same-frame, gdb-find-source-frame)
(breakpoint-enabled, breakpoint-disabled, gdb-max-frames)
(gdb-all-registers, gdb-memory-repeat-count, gdb-memory-format)
(gdb-memory-unit): Move to new group from GUD group.
(menu): Allow customization from GDB-UI menu-item.