Drop the libxfce4windowing vapi and restrict to using 4.20.4 and later

[fossfreedom]

As per BuddiesOfBudgie/budgie-desktop@7396277

We don't need to have our own vapi - and potentially breakages if we don't update the vapi with upstream changes. Newer libxfce4windowing ships with its own vapi.

The change is that all references to libxfce4windowing in the codebase needs to change to Xfw (see the above commit).

EDIT: For debian we have to wait until libxfce4windowing changes to meson and generates the vapi