tokio recommends using a sync mutex with a notifier instead of the
async mutex where possible. Rust forbids holding a sync mutex guard
across await points so we can prevent a potential deadlock this way.
This adds a custom channel based on the tokio mpmc example for
handling gvfs events from callbacks to avoid the async mutex
requirement. Messages are held in a `VecDeque` behind a sync mutex
and the receiver will get notified via the notifier when a message
is added to the queue.
Weak references used in gio callbacks in case the sender is dropped
by the application.
