gnucash maint: Register speed-up for large files.

[Christian Stimming]

Updated	 via  https://github.com/Gnucash/gnucash/commit/92ea3ba8 (commit)
	from  https://github.com/Gnucash/gnucash/commit/f144a8de (commit)



commit 92ea3ba8a60bf4eb19d9b6932fb3ed8b582551a5
Author: Christian Stimming <christian at cstimming.de>
Date:   Fri Jun 22 22:17:38 2018 +0200

    Register speed-up for large files.
    
    The function qof_book_use_split_action_for_num_field gets called quite a
    lot in each register display refresh (due to sorting all splits from
    Split.x's xaccSplitOrder function), but it always used to use a KVP
    lookup, which is rather expensive compared to accessing a gboolean member
    variable.
    
    To get rid of this cost, I had to remove the KVP lookup in this
    simple-looking function. The pattern is this: A gboolean cache variable is
    introduced, along with an isvalid flag. The lookup makes the expensive
    KVP lookup once, then caches the value. The GObject property mechanism
    offers a callback for when the setter was called, which is used to mark
    the cached value as invalid. A parallel setter method (here:
    qof_book_set_option) also just marks the cache as invalid. This covers
    all setters, and the getters will use the cached value except for their
    first invocation.
    
    The NUM_FIELD_SOURCE feature was introduced in 2012 by the very large
    commit 7cdd7372 and apparently its costs never were a problem
    until the KVP lookup became more costly due to the std::vector
    construction and destruction.



Summary of changes:
 libgnucash/engine/qofbook.cpp | 63 +++++++++++++++++++++++++++++++++++++------
 libgnucash/engine/qofbook.h   |  7 +++++
 2 files changed, 62 insertions(+), 8 deletions(-)