[GNC] New price source

[Christopher Lam]

To casual and regular users:
In addition to pricedb-nearest and pricedb-latest, there'll be a new price
source 'pricedb-nearest-before' in 4.6 onwards; this will ignore prices
*after* the report date.
e.g. a user records weekly prices e.g. on 19-dec, 26-dec, 2-jan, 9-jan etc
and runs the balance sheet report dated 31-dec.
Although the nearest price is 2-jan, we'd usually want the 26-dec price
instead (e.g. a stock-split, or a GFC may have occurred on 1-jan).
Please test and report -- recent builds 24 April 2021 onwards have this new
source.
https://code.gnucash.org/builds/flatpak/maint/ and
https://code.gnucash.org/builds/win32/maint/
Relevant bug report: https://bugs.gnucash.org/show_bug.cgi?id=743753