Bugfix: Use both parts of WM_CLASS (it contains instance and class) (Thanks fallen)

Actually, WM_CLASS contains two null-terminated strings, so we cannot
use asprintf() to get its value but rather use strdup() to get both
of them. Both values are compared when a client is matched against
a wm_class/title combination (for assignments for example).

include/data.h

@@ -378,8 +378,10 @@ struct Client {
          * in. If set to true, legacy window names are ignored. */
         bool uses_net_wm_name;
 
-        /** Holds the WM_CLASS, useful for matching the client in commands */
-        char *window_class;
+        /** Holds the WM_CLASS (which consists of two strings, the instance
+         * and the class), useful for matching the client in commands */
+        char *window_class_instance;
+        char *window_class_class;
 
         /** Holds the client’s mark, for vim-like jumping */
         char *mark;