4.16.0

- Bump glib to 2.50 based on xfce4.16 roadmap
- Do not process @LINGUAS@ at xdt-autogen stage
- xfce-build: Add Thunar to container (Fixes #40)
- helpers: Add xfce-build 'pull' argument
- Fail xfce-build container generation if there is any error
- helpers: Prefer xfce-build over xfce-test (Fixes #38)
- helpers: Add new helper xfce-open-gitlab
- Disable CFLAGS override when building CI container
- use strict CFLAGS to catch common mistakes in ci pipeline
- helpers: Add helper to locally run xfce-build
- Add xvfb on xfce-build