Maybe adding to addons isn't too bad
You need to talk to @eht16 about that, its his plugin.
I wouldn't say it is "my" plugin, I just wrote it initially :D. while the Addons plugin could catch this feature as well (as it was said, it's already a catch all plugin and maybe already too big), I think the menubar feature fits rather in core Geany because it would align cleanly with the other show/hide options for interface elements like Message Window, Toolbar and Sidebar.
But don't break it, its the only one I have enabled most of the time,
Heh, the best reason to not break it: Lex is using it :D