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/outputs.h

@@ -4,7 +4,6 @@
 #include <xcb/xcb.h>
 
 #include "common.h"
-#include "workspaces.h"
 
 typedef struct i3_output i3_output;