Support for multiple account types for an account placeholder

[Mikael Andersson]

I am quite sure I have read a discussion a while ago about the
constraint of not being able to set more than one account type for an
account. The thing is that the standard BAS account plan, which is being
used in Sweden, has a top account (placeholder) which holds both equity
and liabilities. Its child accounts are split into accounts of either
the types equity or liabilities.

Could this support be implemented without "too much" work? If you think
it seems to be an OK idea but are not interested in implementing it, I
can maybe do it, provided I get some help.

Thanks,

Mikael