wayland: Make GdkWaylandDisplay public

Allows to access Wayland specific display information like wl_display
etc.

Add gdk_wayland_display_get_wl_display for getting the Wayland
wl_display.

gdk/wayland/Makefile.am

@@ -2,6 +2,7 @@
 include $(top_srcdir)/Makefile.decl
 
 libgdkincludedir = $(includedir)/gtk-3.0/gdk
+libgdkwaylandincludedir = $(includedir)/gtk-3.0/gdk/wayland
 
 AM_CPPFLAGS =					\
 	-DG_LOG_DOMAIN=\"Gdk\"			\
@@ -36,4 +37,7 @@ libgdk_wayland_la_SOURCES =			\
 libgdkinclude_HEADERS =				\
 	gdkwayland.h
 
+libgdkwaylandinclude_HEADERS = 			\
+	gdkwaylanddisplay.h
+
 -include $(top_srcdir)/git.mk