Create is_notify_enabled function

The same code was used on two different occasions. I put it into a
function.

src/common.h

@@ -128,5 +128,6 @@ int get_next_available_win_num(GList *used);
 
 char* get_file_or_linked(char *loc, char *basedir);
 char * strip_arg_quotes(const char * const input);
+gboolean is_notify_enabled(void);
 
 #endif