Avoid treating non-X GDK display names as X ones

When running on Wayland, gdk_display_get_name() can return things like
"wayland-0" rather than valid X display names.  PuTTY nonetheless
treated them as X display names, meaning that when running under
Wayland, pterm would set DISPLAY to "wayland-0" in subprocesses, and
PuTTY's X forwarding wouldn't work properly.

To fix this, places that call gdk_display_get_name() now only do so on
displays for which GDK_IS_X_DISPLAY() is true.  As with
GDK_IS_X_WINDOW(), this requires some backward-compatibility for GDK
versions where everything is implicitly running on X.

To make this work usefully, pterm now also won't unset DISPLAY if it
can't get an X display name and instead will pass through whatever value
of DISPLAY it received.  I think that's better behaviour anyway.

There are two separate parts of PuTTY that call gdk_display_get_name().
platform_get_x_display() in unix/putty.c is used for X forwarding, while
gtk_seat_get_x_display() in unix/window.c is used used for setting DISPLAY
and recording in utmp.  I've updated both of them.

unix/putty.c

@@ -18,6 +18,10 @@
 
 #include "gtkcompat.h"
 
+#ifndef NOT_X_WINDOWS
+#include <gdk/gdkx.h>
+#endif
+
 /*
  * Stubs to avoid pty.c needing to be linked in.
  */
@@ -62,8 +66,11 @@ const bool dup_check_launchable = true;
 
 char *platform_get_x_display(void) {
     const char *display;
+#ifndef NOT_X_WINDOWS
     /* Try to take account of --display and what have you. */
-    if (!(display = gdk_get_display()))
+    if (!GDK_IS_X11_DISPLAY(gdk_display_get_default()) ||
+        !(display = gdk_get_display()))
+#endif
         /* fall back to traditional method */
         display = getenv("DISPLAY");
     return dupstr(display);