DATAMATRIX: add new options DM_B256_START and DM_C40_START to

`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

docs/manual.txt

@@ -4143,6 +4143,11 @@ placement, use option --dmiso144 (API option_3 |= DM_ISO_144).
 For a faster but less optimal encodation, the --fast option (API
 input_mode |= FAST_MODE) may be used.
 
+The encodation mode of initial input characters may be forced using the --dmb256
+(API option_3 |= DM_B256_START) and --dmc40 (API option_3 |= DM_C40_START)
+options. Both take an integer (API option_1) which specifies how many initial
+characters to encode in the respective mode, with 0 meaning all.
+
 Data Matrix supports Structured Append of up to 16 symbols and a numeric ID
 (file identifications), which can be set by using the --structapp option (see
 4.17 Structured Append) (API structapp). The ID consists of 2 numbers ID1 and
@@ -5321,6 +5326,16 @@ OPTIONS
     Send output to stdout, which in most cases should be re-directed to a pipe
     or a file. Use --filetype to specify output format.
 
+--dmb256=INTEGER
+
+    For Data Matrix symbols, encode the first INTEGER input characters in Base
+    256 mode. 0 means all input.
+
+--dmc40=INTEGER
+
+    For Data Matrix symbols, encode the first INTEGER input characters in C40
+    mode. 0 means all input.
+
 --dmiso144
 
     For Data Matrix symbols, use the standard ISO/IEC codeword placement for 144