Join translated string to include comment in po file

When _( is on separate line, xgettext does not include comment from
previous line in po file.

Fixes #273.

trans/widgets.py

@@ -261,10 +261,7 @@ class NormalWidget(Widget):
                 '%(percent)d%% complete, help us improve!'
             ),
             # Translators: please use your language name instead of English
-            _(
-                'translating %(count)d strings into English\n'
-                '%(percent)d%% complete, help us improve!'
-            ),
+            _('translating %(count)d strings into English\n%(percent)d%% complete, help us improve!'),
             'Sans', 8,
             72, 22
         )