Add statement of cash flows

[einhverfr]

Moved from https://sourceforge.net/p/ledger-smb/feature-requests/199/

With the new account hierarchy reporting system we should be able to select hierarchies for the sections of the reports properly. For 1.4 and 1.5 this could be report-time selection. At some point we could allow system configuration for hierarchies and this report.

[ehuelsmann]

Would be nice to set up a list of requirements to fulfill https://corporatefinanceinstitute.com/resources/knowledge/accounting/statement-of-cash-flows/