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/child.h

@@ -0,0 +1,9 @@
+#ifndef CHILD_H_
+#define CHILD_H_
+
+#define STDIN_CHUNK_SIZE 1024
+
+void start_child(char *command);
+void kill_child();
+
+#endif