Deprecate GtkEntryCompletion

We want to drop cell renderers and tree models
in GTK 5. The functionality of GtkEntryCompletion
may be replaced by a new widget in GTK 5.

tests/testentrycompletion.c

@@ -21,6 +21,8 @@
 #include <string.h>
 #include <gtk/gtk.h>
 
+G_GNUC_BEGIN_IGNORE_DEPRECATIONS
+
 /* Don't copy this bad example; inline RGB data is always a better
  * idea than inline XPMs.
  */