bug#9350: 23.3; File menu should contain "split window horizontally C-x 3"

[martin rudalics]

> Splitting windows horizontally becomes more and more important as the
> screen sizes rise - and it is prevalent in todays programming
> environments.
>
> So I think that it would be important to add that entry to the menu -
> also because a new user just asked me that, so this is no theoretical
> construct, but an observation.
>
> This would clear up which kind of splitting is meant by the simple
> "split window".

I added an entry for C-x 3.  Please have a look.

martin

Note: Adding an entry for `split-window-sensibly' doesn't strike me as a
good idea since that function is part of the buffer display routines and
constrained by `split-height-threshold' and `split-width-threshold'.