Provide for easily moving GValues to and from KVP

So that all interaction with KVP is easily handled with properties.
Includes convenience functions for simplifying set/get property
functions.
Make most QofInstance functions private.

Import qofinstance-p.h into QofInstance child classes.
Wrap g_object_get/set with qof_instance_get/set

qof_instance_set marks dirty and will eventually also begin and commit edits.
Remove unused functions.
Move the capital gains account creation from cap-gains.c to Account.c

Functions which change an object's state should be members of the objects class.
Remove convenience functions which return a pointer to kvm_data.
Add Account property "lot-next-id"

Replaces direct kvp access.
Add Transaction property "invoice"

Replaces direct KVP access.
Add properties "invoice", "owner-type", and "owner-guid"

Replaces direct KVP access.
Add xaccTransCopyNoKvm

To avoid having to expose the KVM pointer in order to clear it.
Also rename xaccDupeTransaction to dupe_trans and make it static,
because it doesn't commit the new transaction.
Replace xaccSplitCopy with xaccSplitCopyNoKvp and xaccSplitCopyKvp

Provides ability to copy a split without its KVP instead of having to directly access the KVP and replace it with an empty frame.
Remove kvp-data as a QofInstance object property

Not actually used anywhere, but yet another back door into private data.
New Split properties sx-account, sx-credit-formula, sx-debit-formula
Move features hash creation to QofBook

So that the KVP stays private.
Remove QofBook functions qof_book_get_slots & qof_book_kvp_changed
Use online-id properties of Account, Transaction, & Split
Transfer the account-matching functions from import-export to Account

I would have preferred to separate the data-retrieval from the actual
bayesian routines but I didn't think that I could do so without losing
backwards data compatibility.
Add kvp properties sx-credit-numeric, sx-debit-numeric, ax-shares, and online-id to Split.
Add several kvp properties to Account

online-id, ofd-income-account, an-account-id, an-bank-code, ab-account-uid, ab-trans-retrieval
Add Transaction kvp properties from-sched-xaction, online-id.