Make workspace_layout handle all cons at workspace level, not only the first one (+test)

This makes opening new windows on workspace level and moving windows to the
right/left more like in the old i3.

include/con.h

@@ -7,7 +7,7 @@
  * X11 IDs using x_con_init().
  *
  */
-Con *con_new(Con *parent);
+Con *con_new(Con *parent, i3Window *window);
 
 /**
  * Sets input focus to the given container. Will be updated in X11 in the next

include/tree.h

@@ -24,7 +24,7 @@ void tree_init();
  * Opens an empty container in the current container
  *
  */
-Con *tree_open_con(Con *con);
+Con *tree_open_con(Con *con, i3Window *window);
 
 /**
  * Splits (horizontally or vertically) the given container by creating a new

include/workspace.h

@@ -109,4 +109,16 @@ void workspace_update_urgent_flag(Con *ws);
  */
 void ws_force_orientation(Con *ws, orientation_t orientation);
 
+/**
+ * Called when a new con (with a window, not an empty or split con) should be
+ * attached to the workspace (for example when managing a new window or when
+ * moving an existing window to the workspace level).
+ *
+ * Depending on the workspace_layout setting, this function either returns the
+ * workspace itself (default layout) or creates a new stacked/tabbed con and
+ * returns that.
+ *
+ */
+Con *workspace_attach_to(Con *ws);
+
 #endif