Motivation for this change

The latest version of XMind doesn't support a prebuilt debian package
anymore, hence a manual installation of the ZIP is needed. The new
configuration for the Java GUI had to be patched as it relied on several
paths in the ZIP that won't work anymore in a Nix-based environment.

Additionally the following changes were made:

• Manual creation of a desktop item and an icon (we're using the one
  from AUR for now). Those files were available in the old .deb, but
  that isn't supported for the latest XMind version.

• Created a patch fro XMind.ini to use writable paths in $HOME and fix
  the path to the plugins in the store path.

• Created a custom startup script which ensures the creation of
  $HOME/.xmind and copies the relevant files from the store path into
  it. Such a behavior was available in the old .deb as well, but not
  anymore with the new ZIP.

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.