Add meta_window_is_remote

It's useful for plugins to be able to easily detect whether
or not a window is from a remote host.  Also, make use of this
in the window delete codepath, instead of looking up the hostname
each time.

https://bugzilla.gnome.org/show_bug.cgi?id=620585

src/core/display-private.h

@@ -83,10 +83,11 @@ struct _MetaDisplay
   
   char *name;
   Display *xdisplay;
+  char *hostname;
 
   Window leader_window;
   Window timestamp_pinging_window;
-  
+
   /* Pull in all the names of atoms as fields; we will intern them when the
    * class is constructed.
    */