NSToolbar is off by default in GNUstep and on by default on macOS

[gcasa]

When you add a toolbar as in this example:

https://github.com/gcasa/NSMenuToolbarItem_test

You must do the following on GNUstep:

[toobar setVisible: YES];

for the toolbar to show. On macOS this step is not needed. This is a behavioral difference, not necessarily a bug. I am documenting it here so that we can consider aligning the behavior with macOS.