bug#11880: 24.1.50; `even-window-heights' should be nil by default

[Dmitry Gutov <dgutov@yandex.ru>]

As mentioned in http://debbugs.gnu.org/cgi/bugreport.cgi?bug=11810,
I believe that the behavior enabled by this variable is non-intuitive
and inconsistent.

Many users have become used to it, but looks like the majority just
doesn't know how to disable it. Example:
http://stackoverflow.com/questions/4716855/how-can-i-prevent-emacs-resizing-my-windows

A question to people who like it as it is: what's the idea behind the
window position limitations? Why only resize when the used window is
vertically adjacent to the selected, and only when below it?
Why not resize independent of position, as long as the used window is inside
some vertically split combination?

As alternative to disabling it by default, I propose to
1) Make `window--even-window-heights' resize any window as long as that
would only move the window(s) in the same combination (or something like
that).
2) Restore the original window height on `quit-window'. This amounts to
the following code at the end of `display-buffer-user-some-window':

   (when window
     (prog1
       (window--display-buffer buffer window 'reuse)
       (window--even-window-heights window)))

--Dmitry

GNU Emacs 24.1.50.1 (i386-mingw-nt6.1.7601) of 2012-07-06