`options_3`, allowing forcing of Base 256 or C40 mode a la BWIPP
for initial no. of characters specified in `option_1`, with 0
meaning all (CLI "--dmb256=" and "--dmc40=", GUI also (apart
from MAILMARK_2D, which may be added later);
export masks `DM_B256_C40_START_MASK` & `DM_SQUARE_DMRE_MASK` in
"zint.h"
ZBarcode_Encode_File: report filename (possibly truncated) in error
message on failed open
GUI: uniquify some accelerators and add some child widget getter
helpers to "mainwindow.cpp"
backend/DEVELOPER -> backend/README, with some expansion
debian/copyright: a few more fixes
BWIPP/pandoc: update to latest
tests/fuzz: adjust for new DATAMATRIX options
f9a493522f