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

frontend_qt/grpCodablockF.ui

@@ -348,7 +348,7 @@
      <item row="1" column="0">
       <widget class="QLabel" name="lblCbfHeight">
        <property name="text">
-        <string>Number of Ro&amp;ws:</string>
+        <string>Nu&amp;mber of Rows:</string>
        </property>
        <property name="toolTip">
         <string>Set number of rows</string>
@@ -605,7 +605,7 @@ the Appearance tab is not checked
 (ignored if disabled)</string>
        </property>
        <property name="text">
-        <string>Row &amp;Height:</string>
+        <string>Ro&amp;w Height:</string>
        </property>
        <property name="alignment">
         <set>Qt::AlignLeading|Qt::AlignLeft|Qt::AlignVCenter</set>
@@ -777,7 +777,7 @@ separating rows</string>
       <item>
        <widget class="QRadioButton" name="radCbfHIBC">
         <property name="text">
-         <string>H&amp;IBC</string>
+         <string>&amp;HIBC</string>
         </property>
         <property name="toolTip">
          <string>Process data as a Health Industry Barcode (HIBC)