Fix _placeholder validator and auto fix them and _displayName

The output of placeholder validator is not in the same format as _displayname and is more confusing. Moreover it asks for placeholders for enum that is not expected/used.

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The fix also include a feature to automatically add missing _displayName and _placeholder in Messages.properties files.

To automatically fix _placeholder and _displayname set this property: -Dtalend.validation.internationalization.autofix=true