cache translations in dpgettext() builtin

do not load the translations again in every call,
check if the translations have been already loaded

fixes very slow start in yast ncurses menu
(yast/ycp-killer#467)

src/ruby/ycp/builtins.rb

@@ -681,7 +681,16 @@ def self.dngettext (domain, singular, plural, num)
     # Translates the text using the given text domain and path
     def self.dpgettext (domain, dirname, text)
       old_text_domain = FastGettext.text_domain
-      FastGettext.add_text_domain(domain, :path => dirname)
+
+      # remember the domain => file mapping, the same domain might be
+      # used from a different path, then we need to reload the translations
+      @textdomain_mapping ||= {}
+
+      # check if the domain is already loaded from the path
+      if @textdomain_mapping[domain] != dirname && !FastGettext::translation_repositories[domain]
+        FastGettext.add_text_domain(domain, :path => dirname)
+        @textdomain_mapping[domain.dup] = dirname.dup
+      end
       FastGettext.text_domain = domain
       return FastGettext::Translation::_(text)
     ensure