This eases overrides by other addons.

Don't always clear the account out to play better with other addons that may
set both "account" & "account type" fields at once.

- More flexible from-to selectors.
- The period filter is now a from-to selector.
- Always preview matching references.
- Add a matched reference count.

The new ``reporting_balance`` field is a simple ``debit - credit`` op.

Update the version to 8.0.1.2.0 (a mig script creates the new column).