layout restoring: append at the nearest split container (or workspace) (Thanks chris)

Before this commit, leaf containers (such as terminal emulators) would
get children appended, which is not intended.

fixes #1223

include/load_layout.h

@@ -10,4 +10,4 @@
  */
 #pragma once
 
-void tree_append_json(const char *filename, char **errormsg);
+void tree_append_json(Con *con, const char *filename, char **errormsg);