Updates to meta-slint

[tronical]

Hi,

This update concerns everyone using meta-slint to build Slint for C++ within Yocto.

The repo's recipes used to depend on either Yocto's built-in rust/cargo support or optionally the meta-rust layer.

Unfortunately, over time has has become difficult to maintain support for the recipes across different Yocto versions as the Rust interfaces change slightly and more importantly it became harder to back-port newer Rust versions into older Yocto releases. Therefore, meta-slint as of today uses meta-rust-bin from the official Rust on Embedded Devices Working Group to satisfy's Slint's internal need for Rust and Cargo for building. This layer provides the latest versions of Rust in a way that work well with different (older) Yocto releases and they're quick to use as they recipes in the layer re-use the official rust/cargo binaries.

If you're using meta-slint and pull, you'll need to make changes to your layer configuration:

1. Remove the meta-rust layer.
2. Add meta-rust-bin.

Let me know if you run into any issues.