Please make the `xdg-portal`/`gtk3` features additive

[sin-ack]

In our project, we want to propagate the choice of RFD's xdg-portal and gtk3 backend on Linux to users (since we observed that for some users xdg-portal fails to launch a file dialog, and GTK3 is too heavy for others). However, this causes issues with running Clippy/tests with --all-features enabled, because rfd fails when both feature flags are enabled:

error: failed to run custom build command for `rfd v0.17.2`

Caused by:
  process didn't exit successfully: `/home/agni/repos/.../target/debug/build/rfd-8c26301fe9ace4c3/build-script-build` (exit status: 101)
  --- stderr

  thread 'main' (23589) panicked at /home/agni/.local/share/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rfd-0.17.2/build.rs:14:17:
  You can't enable both `gtk3` and `xdg-portal` features at once
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

It would be good if one could be chosen as the default, and a feature could be enabled to select another. This cannot be done from our side because features cannot be used to disable a feature in a dependency.