Update es.po

Use same text in Split Window menu as in Keybindings dialog.
Also, translate "Side by side" as "Izquierda y derecha" ("left and right") rather than "Yuxtapuestos" ("juxtaposed") which is a rather weird (and inaccurate) word.
Merge pull request #382 from cousteaulecommandant/patch-3

Update es.po