Improve tab behaviour in Stock split assistant

This is a follow-up on Bug 650584 - Change Stock Split Druid to an Assistant
With these changes the amount edits in the assistant react to normal focus
events using keyboard navigation.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23188 57a11ea4-9604-0410-9ed3-97b8803252fd