Add menu bar with julia-snail commands

[rudi]

Hi, I keep forgetting the key bindings for interacting with julia-snail; this pull request adds a Julia-Snail menu bar with most commands defined in julia-snail-mode-map.

[gcv]

I took the liberty of renaming the menu titles so they take a little less space in the menu bar. But this is great. Thank you!

[rudi]

Great, thanks for merging!