ci: appimage: Add build job, Drop fuse dependency

By default it now will use /usr as Qt install path. Also the fuse
dependency isn't needed anymore (didn't work in docker); we're just
extracting the image now (thanks to JBB).
6 jobs for master in 5 minutes and 24 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #5260
archlinux

00:00:34

passed #5258
click-xenial

00:01:07

passed #5261
debian-sid

00:01:15

passed #5259
linux-appimage

00:01:34

passed #5262
neon-bionic

00:00:34

passed #5263
ubuntu-trusty

00:00:33