Motivation for this change

If dropbox can't load libappindicator, it will fallback to using the old
xembed protocol. If you (or your Desktop Environment) are running
something like KDE's XEmbed SNI Proxy, then you'll at least see an icon,
but clicking the icon won't give you menu.

Adding libappindicator to the env allows dropbox to fully integrate with
contemporary status trays, while still falling back to xembed if the
required DBus service is not present.

This should also be backported to (at least) the nixos-19.03 channel.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.