[3.0] Ignore model constants in platform classes in generating models (parse-schema)

[Mark-H]

Given a model class like..

namespace Namespace\Model;
class SomeObject extends xPDOObject
{
    protected const SOME_CONSTANT = 'some relevant value';

... the created platform classes get:

namespace Namespace\Model\mysql;
class SomeObject extends \Namespace\Model\SomeObject
    const SOME_CONSTANT = 'some relevant value';

... which I don't necessarily want. Being able of disabling that (or, at the very least, keeping the same visibility) would be nice.

Related to #162 which fixed the constant values getting skipped entirely, which has since improved, but is still not ideal behavior in my humble opinion.