Unsaved changes prompt

[b10n]

When closing a glrnvim window holding a buffer with unsaved changes, neovim gets terminated and a swap file remains.
Instead, it would be more graceful to have a "Save changes" prompt first, e.g. like neovide.

[beeender]

This would be difficult to support. Since the terminal gets the quit signal but not the neovim. There is no general way to hook with the terminal's exit function.