Ensure that opening an existing book never opens an empty main window.

If there's only one window, automatically add an account hierarchy page
to it. If it's a subsequent empty window, just ignore it (ie don't restore
it from the state file).

This commit deals with the secondary issue mentioned in
bug 436479 - Gnucash doesn't display the data file name in the title bar

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23528 57a11ea4-9604-0410-9ed3-97b8803252fd