Efl.Ui.Panes: add first and second as content aliases

'first' and 'second' are more meaningful than
'left'/'right' & 'up'/'down' names.

refer T5359

src/lib/elementary/efl_ui_panes.c

@@ -37,10 +37,13 @@ static const Evas_Smart_Cb_Description _smart_callbacks[] = {
 
 static const Elm_Layout_Part_Alias_Description _content_aliases[] =
 {
+   //XXX: change elm.swallow.left/right to *.first/second in new theme.
    {"left", "elm.swallow.left"},
    {"right", "elm.swallow.right"},
    {"top", "elm.swallow.left"},
    {"bottom", "elm.swallow.right"},
+   {"first", "elm.swallow.left"},
+   {"second", "elm.swallow.right"},
    {NULL, NULL}
 };