appimage: Fix build; Use QXmpp from system

It also adds the zsync file to automatically update the AppImage.
6 jobs for master in 4 minutes and 51 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #5405
archlinux

00:01:18

passed #5403
click-xenial

00:02:28

passed #5407
debian-sid

00:01:54

passed #5404
linux-appimage

00:03:32

passed #5406
neon-bionic

00:03:18

passed #5408
ubuntu-trusty

00:01:57