Deleting an account referenced by other objects

[Phil Longstaff]

Bug 140400 (https://bugzilla.gnome.org/show_bug.cgi?id=140400) says that
you can delete an account even though business objects still refer to
that account.  I want to tackle this bug.

In general, no object should be deleted if there are other objects that
refer to it.  What do we want gnucash to do if the user tries to do
this?

If I try to delete an account that still has splits, I am given a dialog
box so that you can move the splits to another account.  Suppose I try
to delete a tax table that still has vendors or billterms that refer to
it?  Get an option to move them to a new tax table?  Simplest response
would be to show me a list of objects that have references ("Customer
ABC, Vendor V, ...") and let me clean them up, only allowing the delete
to go ahead once the references were gone.  Not as user-friendly
perhaps, but easier to implement given the number of different object ->
object references.  Also, given that theoretically, new objects with
links to existing object types can be added providing a list may be the
best that can be done.

Phil