Automatically delete the client when the proxy is destroyed

Automatically call the new DeleteClient API when the client proxy is
destroyed. This means that application can simply unref their
GClueSimple/GClueClient to correctly clean up everything.

Closes: #102
4 jobs for <span class="ref-name">benzea/auto-close-client</span> in 8 minutes and 51 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #95481
ubuntu-16.04

00:04:34

passed #95458
ubuntu-17.10

00:04:42

passed #95459
ubuntu-18.04

00:05:09

failed #95457
ubuntu-16.04

00:02:31