Support msgctxt in po files instead of a vertical bar in the msgid

For developers, an extra optional context parameter has been
added to the gettext functions. For example, _('point size|pt')
should now be written as _('pt', 'point size').

For translators this would appear in the message catalog as:

msgctxt "point size"
msgid "pt"
msgstr "pt"

Implements #3416.

po/update_po.py

@@ -426,7 +426,8 @@ def retrieve():
     os.system('''%(xgettext)s -F -c -j --directory=./ -d gramps '''
               '''-L Python -o gramps.pot --files-from=python.txt '''
               '''--debug --keyword=_ --keyword=ngettext '''
-              '''--keyword=_T_ --keyword=trans_text '''
+              '''--keyword=_T_ --keyword=trans_text:1,2c '''
+              '''--keyword=_:1,2c --keyword=_T_:1,2c '''
               '''--keyword=sgettext --from-code=UTF-8''' % {'xgettext': xgettextCmd}
              )