#10206 add pos_horz parameter to addIcon method

As part of commit 6999de5 I modified the existing offset property in the
addIcon method, but it would appear that a similar change would have to
be made in ckbuilder.jar because after a build and load
CKEDITOR.skin.icons contains icons objects that only had an "offset"
value, and not the new "offset_vert" value that I had created.

I reverted back to using the "offset" object property, such that calls
to getIconStyle() would correctly handle those values registered in
CKEDITOR.skins.icons[] during "building" by ckbuilder.jar.

Without an additional "override" property, that allows non-"built" skins
that are loaded via:
extraPlugins:'xyz',
skin:'albani'

...at runtime, this change may seem pointless, but one step at a time,
as I'm submitting another change to add the "override" property to allow
runtime loading of a new skin that wasn't "built" into ckeditor.js