Minor library identifier validator change.

Use the wxTextValidator style flag to allow the user to clear the library
identifier string.  Use wxTextValidator::SetStyle( wxFILTER_EMPTY ) to
prevent empty library identifier strings which is the current behavior.

(cherry picked from commit 9f9e87f11f)

common/validators.cpp

@@ -1,7 +1,7 @@
 /*
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
- * Copyright (C) 2013 Wayne Stambaugh <stambaughw@verizon.net>
+ * Copyright (C) 2013 Wayne Stambaugh <stambaughw@gmail.com>
  * Copyright (C) 2004-2019 KiCad Developers, see change_log.txt for contributors.
  * Copyright (C) 2018 CERN
  *
@@ -291,6 +291,10 @@ bool LIB_ID_VALIDATOR::Validate( wxWindow *aParent )
     wxString val( text->GetValue() );
     wxString tmp = val.Clone();          // For trailing and leading white space tests.
 
+    // Allow empty string if empty filter not set to allow clearing the LIB_ID.
+    if( !(GetStyle() & wxFILTER_EMPTY) && val.IsEmpty() )
+        return true;
+
     if( tmp.Trim() != val )              // Trailing white space.
     {
         msg = _( "Entry contains trailing white space." );