Check for horizontally-split windows when shrinking a window.

Check for horizontally-split windows when shrinking a window.

So far, Org used either `fit-window-to-buffer' or
`shrink-window-if-larger-than-buffer' without any further checks when
displaying one of its many help and selection buffers.  This can cause
problems if the user has set up Emacs to split windows horizontally
rather than vertically, because the window being shrunken then may be
side-by-side with another window, and shrinking the height of one will
also change the other.

With this patch, shrinking a window always goes through the new
function `org-fit-window-to-buffer' which only acts if the current
window spans the whole width of the frame.

Furthermore, this function also helps with compatibility, because it
falls back to `shrink-window-if-larger-than-buffer' if
`fit-window-to-buffer' does not exist, as is the case on older version
of Emacs and XEmacs.