Disable CSDs on Wayland by default

[lah7]

In my testing, this worked great for meld 3.20.4 (file browser, about dialog), but had no effect for the main window for gnome-disks 3.38.2.

The workaround of using GDK_BACKEND=x11 as suggested in https://github.com/lah7/gtk3-classic/issues/28#issuecomment-1270508869 works better.

Fixes #28... sorta. Credit to @droc12345 for the patch.