Break up gtkmisc.c.

It's another file that should have been subdivided into lots of tiny
separate things in the utils library - especially since for some
reason I made a completely separate 'guimisc' cmake-level library for
it when there was no need.

unix/utils/get_x11_display.c

@@ -0,0 +1,34 @@
+/*
+ * Return the Xlib 'Display *' underlying our GTK environment, if any.
+ */
+
+#include <gtk/gtk.h>
+#include "putty.h"
+#include "gtkcompat.h"
+#include "gtkmisc.h"
+
+#ifndef NOT_X_WINDOWS
+
+#include <gdk/gdkx.h>
+#include <X11/Xlib.h>
+
+Display *get_x11_display(void)
+{
+#if GTK_CHECK_VERSION(3,0,0)
+    if (!GDK_IS_X11_DISPLAY(gdk_display_get_default()))
+        return NULL;
+#endif
+    return GDK_DISPLAY_XDISPLAY(gdk_display_get_default());
+}
+
+#else
+
+/*
+ * Include _something_ in this file to prevent an annoying compiler
+ * warning, and to avoid having to condition out this file in
+ * CMakeLists. It's in a library, so this variable shouldn't end up in
+ * any actual program, because nothing will refer to it.
+ */
+const int get_x11_display_dummy_variable = 0;
+
+#endif