first step of the big refactoring ("tree" branch).

From here on, we can track changes. It made no sense to put the
development up to this point into git.

include/con.h

@@ -0,0 +1,20 @@
+#ifndef _CON_H
+#define _CON_H
+
+Con *con_new(Con *parent);
+bool con_is_leaf(Con *con);
+bool con_accepts_window(Con *con);
+Con *con_get_output(Con *con);
+Con *con_get_workspace(Con *con);
+Con *con_get_fullscreen_con(Con *con);
+bool con_is_floating(Con *con);
+Con *con_by_window_id(xcb_window_t window);
+Con *con_by_frame_id(xcb_window_t frame);
+Con *con_for_window(i3Window *window, Match **store_match);
+void con_attach(Con *con, Con *parent);
+void con_detach(Con *con);
+
+enum { WINDOW_ADD = 0, WINDOW_REMOVE = 1 };
+void con_fix_percent(Con *con, int action);
+
+#endif