Fix string allocation handling with GtkBuilder translations

Fix string allocation handling with GtkBuilder translations

Use g_string_assign to avoid issues with assigning GString's internal
buffer back to the same string. This can happen when no translations are
available and _gtk_builder_parser_translate returns back the same
pointer that was passed in.

This fixes a regression from commit e9f182e37a7f6e2dc339054841a3c9f930f
that caused GtkComboBoxText <items> from GtkBuilder to show up empty if
no translations are available.

https://bugzilla.gnome.org/show_bug.cgi?id=700629