- Add special symbology-specific escape sequences (Code 128 only)

for manual Code Set switching via `input_mode` flag
  `EXTRA_ESCAPE_MODE` (CLI `--extraesc`) (ticket #204)
- GUI: disable "Reset" colour if default; add "Unset" to Printing
  Scale dialog (allows unsetting of X-dim/resolution settings
  without having to zap)
- library: guard against out-of-bounds rows (negative)
- test suite: fix some clang-tidy warnings; slight coverage
  improvements

frontend_qt/extScale.ui

@@ -302,6 +302,20 @@
    </item>
    <item>
     <layout class="QHBoxLayout" name="horzLayoutScaleBtns">
+     <item>
+      <widget class="QPushButton" name="btnScaleUnset">
+       <property name="autoDefault">
+        <bool>false</bool>
+       </property>
+       <property name="text">
+        <string> &amp;Unset</string>
+       </property>
+       <property name="toolTip">
+        <string>Reset the scale to the original value and
+mark the settings as unset</string>
+       </property>
+      </widget>
+     </item>
      <item>
       <spacer name="horzSpacerScaleBtns">
        <property name="orientation">