Prepare for "Japan's Y2K" Gengou calendar era switch after 2019-04-30

The emperor Akihito will abdicate on 2019-04-30. The next emperor
will be Naruhito, but so far neither the new era name (Heisei for
Akihito) nor its abbreviation or a Unicode character are
determined. At least introduce the new era with some dummy names
(Naruhito,Na,N).

Change-Id: I8c0af390ca0408ac259e47e7eaf2e49b5889c9ba
Reviewed-on: https://gerrit.libreoffice.org/58142Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins

i18npool/source/calendar/calendar_gregorian.cxx

@@ -205,10 +205,11 @@ Calendar_hanja::loadCalendar( const OUString& /*uniqueID*/, const css::lang::Loc
 }
 
 static const Era gengou_eraArray[] = {
-    {1868,  1,  1, 0},
-    {1912,  7, 30, 0},
-    {1926, 12, 25, 0},
-    {1989,  1,  8, 0},
+    {1868,  1,  1, 0},  // Meiji
+    {1912,  7, 30, 0},  // Taisho
+    {1926, 12, 25, 0},  // Showa
+    {1989,  1,  8, 0},  // Heisei
+    {2019,  5,  1, 0},  //(Naruhito) (TODO: real era name not known yet (2018-07-26))
     {0, 0, 0, 0}
 };
 Calendar_gengou::Calendar_gengou() : Calendar_gregorian(gengou_eraArray)

i18npool/source/localedata/data/ja_JP.xml

@@ -480,6 +480,13 @@
           <DefaultAbbrvName>平</DefaultAbbrvName>
           <DefaultFullName>平成</DefaultFullName>
         </Era>
+        <Era>
+          <!-- TODO: Naruhito is only the emperor's name, the era name and
+               abbreviation and full name are not known yet (2018-07-26) -->
+          <EraID>Naruhito</EraID>
+          <DefaultAbbrvName>Na</DefaultAbbrvName>
+          <DefaultFullName>Naruhito</DefaultFullName>
+        </Era>
       </Eras>
       <StartDayOfWeek>
         <DayID>sun</DayID>

svl/source/numbers/zformat.cxx

@@ -3409,6 +3409,9 @@ void SvNumberformat::ImpAppendEraG( OUStringBuffer& OutString,
         case 4:
             cEra = 'H';
             break;
+        case 5:
+            cEra = 'N';     /* TODO: the real one letter era name is not known yet (2018-07-26) */
+            break;
         default:
             cEra = '?';
             break;