Similarly to g_source_set_name(), this sets a name on a GTask for
debugging and profiling. Importantly, this name is propagated to the
GSource for idle callbacks for the GTask, ending the glorious reign of
`[gio] complete_in_idle_cb`.

The name can be queried using g_task_get_name(). Locking is avoided by
only allowing the name to be set before the GTask is used from another
thread.

Includes tests.

Signed-off-by: Philip Withnall <withnall@endlessm.com>