fdo#32368 - Link both Ctrl+Y as Ctrl+Shift+Z as shortcut keys for Redo

As discussed on the UX-advice and bug report itself,
we agreed to link both shortcut keys to the redo command.

Due http://opengrok.libreoffice.org/xref/core/framework/source/inc/accelerators/acceleratorcache.hxx#75 :
/*map commands to keys in relation 1:n. First key is interpreted as preferred one!*/ the shortcut key
that is mentioned in the menu entry is ctrl+y (or cmd+y for Mac users).

UX-advice thread: http://lists.freedesktop.org/archives/libreoffice-ux-advise/2013-May/002033.html

Change-Id: I8d766b300d79e9ebd85dc5ae7bb327cf9c1a7c41
Reviewed-on: https://gerrit.libreoffice.org/3853Reviewed-by: Bosdonnat Cedric <cedric.bosdonnat@free.fr>
Tested-by: Bosdonnat Cedric <cedric.bosdonnat@free.fr>