register window: entire split should be visible

[Ben Stanley]

package: register window
version: CVS March 26

When opeining a split (either in auto-split or basic-ledger and then
manually open the split), the register window should scroll to show the
entire split if at all possible. It would be nice to have at least one
or two normal ledger lines visible above and below the opened split as
well after the scrolling has been done.

In the current situation, if you are manually opening a split view of
the last transaction, you are guaranteed to have to manually scroll the
ledger to be able to see it. This is almost never what you wanted...