Move Serbian po

Updates the Serbian language code from sr_RS to sr.  This appears to
work as intended under Debian whereas the full code, despite existing in
the i18n directory was not recognized by wxWidgets or Weblate.

common/pgm_base.cpp

@@ -92,6 +92,7 @@ LANGUAGE_DESCR LanguagesList[] =
     { wxLANGUAGE_POLISH,     ID_LANGUAGE_POLISH,     wxT( "Polski" ),   true },
     { wxLANGUAGE_PORTUGUESE, ID_LANGUAGE_PORTUGUESE, wxT( "Português" ),true },
     { wxLANGUAGE_RUSSIAN,    ID_LANGUAGE_RUSSIAN,    wxT( "Русский" ),  true },
+    { wxLANGUAGE_SERBIAN,    ID_LANGUAGE_SERBIAN,    wxT( "Српски"),    true },
     { wxLANGUAGE_FINNISH,    ID_LANGUAGE_FINNISH,    wxT( "Suomalainen" ),  true },
     { wxLANGUAGE_VIETNAMESE, ID_LANGUAGE_VIETNAMESE, wxT( "Tiếng việt" ), true },
     { wxLANGUAGE_TURKISH,    ID_LANGUAGE_TURKISH,    wxT( "Türk" ),     true },

include/id.h

@@ -144,6 +144,7 @@ enum main_id
     ID_LANGUAGE_LATVIAN,
     ID_LANGUAGE_LITHUANIAN,
     ID_LANGUAGE_VIETNAMESE,
+    ID_LANGUAGE_SERBIAN,
     ID_LANGUAGE_CHOICE_END,
 
     // Popup Menu (mouse Right button) (id consecutifs)

translation/pofiles/LINGUAS

@@ -24,7 +24,7 @@ pt
 ru
 sk
 sl
-sr_RS
+sr
 sv
 tr
 vi