When editing meta items, modify them rather than rebuiding them

When editing the default sort key or a category's sort key, we would just
build a brand new meta item and replace the original item with it. This
destroys whitespace information tracked in the .internal property though,
so the resulting diffs looked pretty bad.

Instead, use ve.extendObject() to base the new meta item on the old one,
changing only what we need to change and keeping .internal (and
htmlAttributes and anything else that may be hiding in there) intact.

Change-Id: I40f4403ea2f2d13542d2e3c8c53e2d7f79515381
Merge "When editing meta items, modify them rather than rebuiding them"