Motivation for this change

Adding libappindicator as a runtime dependency allows the Electron-based gui app
to have an appindicator in desktop environments that support them (eg,
Gnome with the appindicator support extension).

Also, previous versions had a broken link in bin/ (mullvad-problem-report) that I noticed while filling out the PR checklist. I have added a commit to fix this.

I have tested that this is compatible with #80721 and #70762. In particular, #80721 really needs to be merged.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/) (mullvad-problem-report does not work before or after this change) (now fixed by second commit)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.