Add support for separator menu items

[pkamb]

I would like to add support for NSMenuItem.separator() menu items. Here's an example inside the main app where I have full control over the menu:

However, when the exact same menu construction code is used within the Finder Extension, we instead see blank lines rather than separator items:

These screenshots are in 10.14 Mojave. I will test with Catalina as well.

But a macOS system update to support these items will be required before Service Station can support them.

[pkamb]

other links to this same issue:

• https://github.com/xamarin/mac-samples/issues/100
• https://stackoverflow.com/questions/32692909/findersync-issues-with-sidebar-icon-toolbar-icon-and-context-menu