vapi: Update libxfce4windowing to 4.19.3

[EbonJaeger]

Description

Updates the XFW vapi to 4.19.3, which has some breaking changes.

Fixes #546

Signed-off-by: Evan Maddock maddock.evan@vivaldi.net

Submitter Checklist

• [ ] Squashed commits with git rebase -i (if needed)
• [x] Built budgie-desktop and verified that the patch worked (if needed)

[EbonJaeger]

This will, of course, fail to build if the distro in use does not have libxfce4windowing version 4.19.3.