Use better names for window-next/-prev and window-vchild/-hchild.

Use better names for window-next/-prev and window-vchild/-hchild.

* window.c (Fwindow_vchild): Rename to Fwindow_top_child.
(Fwindow_hchild): Rename to Fwindow_left_child.
(Fwindow_next): Rename to Fwindow_next_sibling.
(Fwindow_prev): Rename to Fwindow_prev_sibling.

* window.el (window-right, window-left, window-child)
(window-child-count, window-last-child)
(window-iso-combination-p, walk-window-tree-1)
(window-atom-check-1, window-tree-1, delete-window)
(window-state-get-1, display-buffer-even-window-sizes): Adapt to
new naming conventions - window-vchild, window-hchild,
window-next and window-prev are now called window-top-child,
window-left-child, window-next-sibling and window-prev-sibling
respectively.