Add spawn_future and spawn_future_local convenience functions

A couple of gtk-rs apps currently use utility functions to make spawning futures less verbose.
See:
- https://docs.rs/gtk-macros/0.3.0/gtk_macros/macro.spawn.html
- https://gitlab.gnome.org/GNOME/loupe/-/blob/main/src/util/mod.rs#L251

Exposing these functions via glib should fulfill that need.

glib/src/manual_functions.rs

@@ -259,3 +259,30 @@ pub fn file_open_tmp(
         }
     }
 }
+
+// rustdoc-stripper-ignore-next
+/// Spawn a new infallible `Future` on the thread-default main context.
+///
+/// This can be called from any thread and will execute the future from the thread
+/// where main context is running, e.g. via a `MainLoop`.
+pub fn spawn_future<R: Send + 'static, F: std::future::Future<Output = R> + Send + 'static>(
+    f: F,
+) -> crate::JoinHandle<R> {
+    let ctx = crate::MainContext::ref_thread_default();
+    ctx.spawn(f)
+}
+
+// rustdoc-stripper-ignore-next
+/// Spawn a new infallible `Future` on the thread-default main context.
+///
+/// The given `Future` does not have to be `Send`.
+///
+/// This can be called only from the thread where the main context is running, e.g.
+/// from any other `Future` that is executed on this main context, or after calling
+/// `with_thread_default` or `acquire` on the main context.
+pub fn spawn_future_local<R: 'static, F: std::future::Future<Output = R> + 'static>(
+    f: F,
+) -> crate::JoinHandle<R> {
+    let ctx = crate::MainContext::ref_thread_default();
+    ctx.spawn_local(f)
+}