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/grpMicroPDF.ui

@@ -82,7 +82,7 @@ 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>
@@ -170,7 +170,7 @@ tab will be re-enabled)</string>
           <string>Set height per row to default value</string>
          </property>
          <property name="text">
-          <string>De&amp;fault</string>
+          <string>Def&amp;ault</string>
          </property>
          <property name="sizePolicy">
           <sizepolicy hsizetype="Preferred" vsizetype="Fixed">
@@ -238,7 +238,7 @@ tab will be re-enabled)</string>
       <item row="0" column="1">
        <widget class="QRadioButton" name="radMPDFHIBC">
         <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)
@@ -375,7 +375,7 @@ Numbers only, in triplets, maximum length 30 (10 triplets)
 (ignored if disabled)</string>
         </property>
         <property name="text">
-         <string>I&amp;D:</string>
+         <string>&amp;ID:</string>
         </property>
         <property name="alignment">
          <set>Qt::AlignLeft|Qt::AlignLeading|Qt::AlignVCenter</set>