implement gettext localization for python

make gnc_path_get_localedir() and GETTEXT_PACKAGE available for python
to access locales. Import gettext module to provide _ as translator
method. Provide null _-method returning english text in case of missing gettext.
Make a lot of messages translatable by adding _-method. Include python
files to create .pot-files.

bindings/python/gnucash_core.py

@@ -28,7 +28,8 @@
 #  @author Jeff Green,   ParIT Worker Co-operative <jeff@parit.ca>
 #  @ingroup python_bindings
 
-import gnucash.gnucash_core_c as gnucash_core_c
+from gnucash import gnucash_core_c
+from gnucash import _sw_core_utils
 
 from gnucash.function_class import \
      ClassFromFunctions, extract_attributes_with_prefix, \
@@ -46,6 +47,24 @@ from gnucash.gnucash_core_c import gncInvoiceLookup, gncInvoiceGetInvoiceFromTxn
     gnc_numeric_create, double_to_gnc_numeric, string_to_gnc_numeric, \
     gnc_numeric_to_string
 
+try:
+    import gettext
+    # install gettext for _-function, needs path to locales
+    _localedir = _sw_core_utils.gnc_path_get_localedir()
+    _translation = gettext.translation(_sw_core_utils.GETTEXT_PACKAGE, _localedir)
+    _ = _translation.gettext
+except:
+    print("\nProblem importing gettext!")
+    import traceback
+    import sys
+    exc_type, exc_value, exc_traceback = sys.exc_info()
+    traceback.print_exception(exc_type, exc_value, exc_traceback)
+    print()
+
+    def _(s):
+        """Null translator function, gettext not available"""
+        return s
+
 class GnuCashCoreClass(ClassFromFunctions):
     _module = gnucash_core_c