add libi3/ipc_connect, use it in i3-config-wizard, i3-input, i3bar

include/libi3.h

@@ -53,6 +53,13 @@ char *sstrdup(const char *str);
  */
 int sasprintf(char **strp, const char *fmt, ...);
 
+/**
+ * Connects to the i3 IPC socket and returns the file descriptor for the
+ * socket. die()s if anything goes wrong.
+ *
+ */
+int ipc_connect(const char *socket_path);
+
 /**
  * Formats a message (payload) of the given size and type and sends it to i3 via
  * the given socket file descriptor.