gtask: Fix a signed/unsigned integer comparison

The GSource times assigned to creation_time are always signed.

https://bugzilla.gnome.org/show_bug.cgi?id=736806
gtask: Document signal handler reference counting

Explain why the signal handler holds a reference to the GTask, even
though that causes a reference loop at first glance.

https://bugzilla.gnome.org/show_bug.cgi?id=736806
gtask: Ignore errors from g_thread_pool_push()

g_thread_pool_push() only returns an error if it fails to spawn a new
thread. However, it unconditionally adds the task to its worker queue,
so:
 • if _any_ threads exist in the pool, the task will eventually be
   handled; and
 • if _no_ threads exist in the pool, the task will be handled if one
   is eventually successfully spawned.
If no more threads are ever spawned, the process probably has bigger
problems than a single GTask which is taking forever to complete.

https://bugzilla.gnome.org/show_bug.cgi?id=736806