Freeciv

I think I can do this one, for qt, gtk3.22, gtk4 & gtk5 clients. But better to wait until #1292 is done because of collisions.

The qt client already has zoom menu items.

It turns out that gtk will not let you use "+" or "-" keys by themselves as menu accelerator keys. In gtk3.22 they just don't work, gtk4 gives an error message. Combining one of them with Ctrl or Meta seems OK, the View menu item Show Stack Size uses Ctrl/Meta and "+". There's really no other menu accelerator keys that are suitable. I could add zoom in and zoom out menu items without menu accelerator keys, but then would want to leave in the current functionality of having "+" or "-" keys do zooming. So wondering if it's worthwhile.