List form factors in webapp desktop files

Fixes Librem5/debs/epiphany#23
4 jobs for web-apps in 10 minutes and 32 seconds (queued for 1 second)
Status Job ID Name Coverage
  Package
passed #302700
librem5
package:deb-debian-buster

00:03:03

passed #302701
librem5:arm64
package:deb-debian-buster:arm64

00:09:51

 
  Test Package
passed #302702
librem5
autopkgtest-debian-buster-package

00:00:37

passed #302703
librem5
lintian-debian-buster-package

00:00:41