Use "C-unwind" API in our vtable

[ogoffart]

When using rust, allow panics to cross the boundaries of our vtable traits. This avoids panic producing two backtrace with panic=unwind

This patch doesn't touch the ABI of out FFI interface, they stay extern "C", because if a panic or exception crosses these boundaries, we are in trouble. (Also, we have a panic=abort in our Cargo.toml anyway)

[ogoffart]

Looks like this causes problem with wasm:

error: failed getting Wasm module for '/home/runner/work/slint/slint/target/wasm32-unknown-unknown/release/slint_wasm_interpreter.wasm'

Caused by:
    0: failed to parse input as wasm
    1: failed to parse code section
    2: exceptions support is not enabled (at offset 0xbd231)
Error: Running the wasm-bindgen CLI
Caused by: Running the wasm-bindgen CLI
Caused by: failed to execute `wasm-bindgen`: exited with exit status: 1
  full command: "/home/runner/.cache/.wasm-pack/wasm-bindgen-46a41f966b4cfc69/wasm-bindgen" "/home/runner/work/slint/slint/target/wasm32-unknown-unknown/release/slint_wasm_interpreter.wasm" "--out-dir" "/home/runner/work/slint/slint/api/wasm-interpreter/pkg" "--typescript" "--target" "web"

(same error for all wasm builds)

2: exceptions support is not enabled (at offset 0xbd231)

Maybe we need to enable them somehow?