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/workspaces.h

@@ -2,7 +2,6 @@
 #define WORKSPACES_H_
 
 #include "common.h"
-#include "outputs.h"
 
 typedef struct i3_ws i3_ws;