Recognition of an unused attribute

[maraf]

Sometimes we need to recognize if attribute is used or not. Until now, such attribute typically had a weird default value and we count on that it will be never used. Minor drawback is that it was shown in the docs.

Here I propose to system wide default attribute value and allow to mark an attribute to default to this value. It must be a constant (probably in PhpRuntime class), so it can be used in php source code as method parameter default value.

<attribute>
    ...
    <default as="unused" />
    ...
</attribute>

[maraf]

XSD deployed.