Add links to constants

[haszi]

Add links to constants with IDs in the following two standard formats (xml:id doesn't have to be on the <constant> element):

<constant xml:id="extension-namespace-or-classname.constants.name-of-constant">
 Extension\Namespace\Or\ClassName::NAME_OF_CONSTANT
</constant>

<constant xml:id="constant.name-of-constant">
 NAME_OF_CONSTANT
</constant>

Please note that there is a large number of constants not using one of these standard formats.

[Girgias]

I think this is a good initial improvement