Figure out callbacks

[mzabaluev]

We need to figure out the semantics of passing safe callbacks from Rust to GObject:

• What type and sigil should the callback closure have.
• Which task it is to be called in.

[mzabaluev]

The callback task could be the global dispatch task from #2.

[mzabaluev]

Callbacks are considered working as of 5d81d4e.
A callback is, naturally, an owned lambda. For GIO async methods, a callback is called in the thread owning the object's affine GMainContext.