interface: Allow marking as sync so signals can be emitted from any t…

[thiblahute]

…hread

Each implementer will the need to mark itself as Send still and everything should be safe.

Without that we get:

thread '<unnamed>' panicked at 'Value accessed from different thread than where it was created', /var/home/thiblahute/.cargo/git/checkouts/gtk-rs-core-7be42ca38bd6361c/4707f68/glib/src/thread_guard.rs:49:9
stack backtrace:
   0: rust_begin_unwind
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/panicking.rs:142:14
   2: glib::thread_guard::ThreadGuard<T>::get_ref
             at /var/home/thiblahute/.cargo/git/checkouts/gtk-rs-core-7be42ca38bd6361c/4707f68/glib/src/thread_guard.rs:49:9
   3: <T as glib::object::ObjectExt>::try_connect_local_id::{{closure}}
             at /var/home/thiblahute/.cargo/git/checkouts/gtk-rs-core-7be42ca38bd6361c/4707f68/glib/src/object.rs:2728:17
   4: <T as glib::object::ObjectExt>::try_connect_unsafe_id::{{closure}}
             at /var/home/thiblahute/.cargo/git/checkouts/gtk-rs-core-7be42ca38bd6361c/4707f68/glib/src/object.rs:2793:27
   5: glib::closure::Closure::new_unsafe::marshal
             at /var/home/thiblahute/.cargo/git/checkouts/gtk-rs-core-7be42ca38bd6361c/4707f68/glib/src/closure.rs:234:26
   6: g_closure_invoke
   7: <unknown>
   8: <T as glib::object::ObjectExt>::try_emit
             at /var/home/thiblahute/.cargo/git/checkouts/gtk-rs-core-7be42ca38bd6361c/4707f68/glib/src/object.rs:2996:13
   9: <T as glib::object::ObjectExt>::emit
             at /var/home/thiblahute/.cargo/git/checkouts/gtk-rs-core-7be42ca38bd6361c/4707f68/glib/src/object.rs:3010:9
  10: <____Object as signals::obj_signals::SignalsExt>::emit_noparam
             at ./tests/signals.rs:1:1
  11: signals::signals::{{closure}}
             at ./tests/signals.rs:84:9
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
thread 'signals' panicked at 'called `Result::unwrap()` on an `Err` value: Any { .. }', tests/signals.rs:87:6
stack backtrace:
   0: rust_begin_unwind
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/panicking.rs:142:14
   2: core::result::unwrap_failed
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/result.rs:1785:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/result.rs:1078:23
   4: signals::signals
             at ./tests/signals.rs:83:5
   5: signals::signals::{{closure}}
             at ./tests/signals.rs:72:1
   6: core::ops::function::FnOnce::call_once
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/ops/function.rs:248:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/a8314ef7d0ec7b75c336af2c9857bfaf43002bfc/library/core/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[thiblahute]

@jf2048 Could you check my latest PR, and what is your plan with that project?