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/common.h

@@ -1,8 +1,6 @@
 #ifndef COMMON_H_
 #define COMMON_H_
 
-#include "util.h"
-
 typedef struct rect_t rect;
 typedef int bool;
 
@@ -17,4 +15,12 @@ struct rect_t {
 	int	h;
 };
 
+#include "queue.h"
+#include "child.h"
+#include "ipc.h"
+#include "outputs.h"
+#include "util.h"
+#include "workspaces.h"
+#include "xcb.h"
+
 #endif