Calling gtk_widget_realize to enforce the existence of an underlying
GdkWindow, followed by gdk_window_ensure_native to enforce an
underlying X window in turn, allows me to get hold of an X window id
on which I can call the Xlib function for setting WM_CLASS, still
before the window is mapped.
With this change, plus Colin's preceding patches, the whole code base
_actually_ compiles and links against GTK 3.22 without any deprecation
warnings. (My claim in commit 8ce237234 that it previously did appears
to have been completely wrong - my guess is that I'd forgotten to
'make clean' before testing against 3.22 and so some source files had
already been compiled against earlier GTK headers.)
af08a7a3b1