Make it easy to create Rust binaries with rpath set to Qt

[tronical]

This originated from #565

^{Originally posted by jamesblacklock October 12, 2021}
I'm getting the following error when I do a cargo run (seems to be just the run; build works fine):

dyld: Library not loaded: @rpath/QtCore.framework/Versions/A/QtCore
  Referenced from: /Users/<snip>/target/debug/budget
  Reason: image not found

Has anyone seen this error or have any familiarity with getting the sixtyfps qt backend working on macos?

When creating a rust binary with cargo and sixtyfps found Qt available for use, we may want to code the rpath to the Qt library/frameworks on Linux and macOS.

The path is initially determined in qttypes's build.rs and printed via

println!("cargo:rustc-cdylib-link-arg=-Wl,-rpath,{}", &qt_library_path);

Unfortunately this has two problems:

1. It only "works" when creating a cdylib, so for example the C++ API of SixtyFPS, but not when creating a Rust binary.
2. The option will cease working in a transitive way as per Tracking issue for rustc-cdylib-link-arg transitive warning rust-lang/cargo#9562

However that cargo issue also suggests a solution that we could implement, which is two fold:

1. We need rustc-link-arg to show up in a stable release of rust/cargo.
2. Inside SixtyFPS we need a reliable way to get hold of the correct rpath and then offer API that applications can call from their build.rs that will emit the correct rustc-link-arg=-Wl,rpath,... text to stdout. This could be done automatically by sixtyfps_build::compile() but perhaps there's also a way to offering either a function for build.rs for apps using the macro or the macro could somehow emit #[link_arg(...)].

[ogoffart]

In the short term, we should document that. At least in https://github.com/sixtyfpsui/sixtyfps/blob/master/docs/install_qt.md

We could make the sixtyfps-backend-qt to also write a rpath somewhere in a file (like we do for SIXTYFPS_DEFAULT_STYLE.txt) and get sixtyfps-build to read it. But that wouldn't work when using the macro.