Be more strict with encapsulation

I.e. move the xcb-event-handling into xcb.c and the
child-process-communications into newly created child.c.
Also change some includes.

i3bar/include/ipc.h

@@ -1,7 +1,6 @@
 #ifndef IPC_H_
 #define IPC_H_
 
-#include <ev.h>
 #include <stdint.h>
 
 int init_connection(const char *socket_path);