Implement tool-bar separators for non-GTK tool-bars.

Implement tool-bar separators for non-GTK tool-bars.

* lisp/tool-bar.el (tool-bar--image-expression): New function.
(tool-bar-local-item, tool-bar--image-exp): Use it.
(tool-bar-setup): Initialize tool-bar-separator-image-expression.
Use :enable instead of :visible to avoid changing the tool-bar
configuration unnecessarily.

* src/keyboard.c (Vtool_bar_separator_image_expression): New variable.
(parse_tool_bar_item): Use it to obtain image separators for
displays not using native tool-bar separators.

* src/xdisp.c (build_desired_tool_bar_string): Don't handle separators
specially, since this is now done in parse_tool_bar_item.

* lisp/info.el (info-tool-bar-map): Add separators.