1. Minor updates: "include what you use"-style cleanups in parser sources
and some dependent files.

2. Major change: the file item_func.h, while being included many times
itself, referenced another massive include file parse_tree_nodes.h for
PT_window.
The current patch moves PT_window into a separate file parse_tree_window.h
to replace parse_tree_nodes.h in item_sum.h.

3. Major change: parse_tree_node.h had a hidden dependency on sql_class.h,
so, the compilation of the former failed if the latest was included after
it.
The current patch moves PT_joined_table::contextualize_tabs() from .h to
the .cc file to stabilize the instantiation of
Parse_tree_node_tmpl<...>::error().

Change-Id: Id949da8ad4c84c0fded311c14a055341197accc8