gnucash maint: Cache current owner balances

[Geert Janssens]

Updated	 via  https://github.com/Gnucash/gnucash/commit/3991ccb9 (commit)
	from  https://github.com/Gnucash/gnucash/commit/40bcd1e3 (commit)



commit 3991ccb9c2383f94315ec647e4250a408bc16ad4
Author: Geert Janssens <geert at kobaltwit.be>
Date:   Sun Sep 23 15:59:27 2018 +0200

    Cache current owner balances
    
    These are queried continuously by the owner tree view (on Customer/Vendor/Employee
    Overview pages) and recalculating them is an expensive operation.
    The cache will be invalidated each time a lot reated to the owner
    changes (modify or delete). The net effect is a huge responsiveness
    improvement of said overviews in case of a large book.



Summary of changes:
 libgnucash/engine/gncCustomer.c  | 93 +++++++++++++++++++++++++++++----------
 libgnucash/engine/gncCustomerP.h |  2 +
 libgnucash/engine/gncEmployee.c  | 93 +++++++++++++++++++++++++++++----------
 libgnucash/engine/gncEmployeeP.h |  2 +
 libgnucash/engine/gncInvoice.c   |  4 +-
 libgnucash/engine/gncOwner.c     | 94 ++++++++++++++++++++++++++++------------
 libgnucash/engine/gncOwnerP.h    |  2 +
 libgnucash/engine/gncVendor.c    | 93 +++++++++++++++++++++++++++++----------
 libgnucash/engine/gncVendorP.h   |  2 +
 9 files changed, 287 insertions(+), 98 deletions(-)