String arguments should be CString

[Supreeeme]

If you pass XdgToplevel::set_title a String with embedded 0 bytes (which is valid for a String), you will get a panic with a backtrace that kinda looks like this:

thread 'main' panicked at ...
xdg.rs:138:5:
called `Result::unwrap()` on an `Err` value: NulError(<bytes>)
stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::result::unwrap_failed
   3: <wayland_protocols::xdg::shell::generated::client::xdg_toplevel::XdgToplevel as wayland_client::Proxy>::write_request
   4: wayland_client::conn::Connection::send_request
   5: wayland_protocols::xdg::shell::generated::client::xdg_toplevel::XdgToplevel::set_title

This is a fairly obtuse error, but digging into the library, it appears this is because string arguments are converted to CStrings internally, with an unwrap:

wayland-rs/wayland-scanner/src/common.rs

Line 541 in df56d7d

vec![quote! { Argument::Str(#arg_name.map(|s| Box::new(std::ffi::CString::new(s).unwrap()))) }]

If wayland-scanner doesn't want to handle this error, then it makes no sense for String arguments to be String, when they should instead just be std::ffi::CString and leave it on callers to deal with constructing it.