Better `quit` configuration

[altsem]

Spawned from: https://github.com/altsem/gitu/issues/255 Relates to: https://github.com/altsem/gitu/issues/252

There's a lot of duplicated bindings config to quit menus. This was done as a workaround to keep other things simpler for a moment. There has to be a better way to specify this. A single binding?

root.quit = ["q", "<esc>"]
...
pull_menu.quit = ["q", "<esc>"]
...
push_menu.quit = ["q", "<esc>"]
...
rebase_menu.quit = ["q", "<esc>"]