Add property linking

[haszi]

Add linking to <property> elements.

This works the same way <constant> tag linking works and is based on the understanding that <property> elements are only to be used for class properties. As with constant linking, property linking requires the <property> elements to include a FQN (eg. <property>DOMNameSpaceNode::$parentElement</property> instead of just <property>$parentElement</property>) and an element with the appropriate xml ID (eg. <varlistentry xml:id="domnamespacenode.props.parentelement">).

Reference issue: #164

[Girgias]

Thank you! Some minor questions and nits, but how does it handle <replaceable> tags? (Because I think how PhD renders function tags which have <replaceable> is quite broken).

[haszi]

Thank you! Some minor questions and nits, but how does it handle <replaceable> tags? (Because I think how PhD renders function tags which have <replaceable> is quite broken).

<replaceable> tags won't work in properties. I thought about making it work but unlike constants, I could not think of a good reason to reference a group of class properties in this way.

On why nested tags are an issue:
PhD is a stream based renderer so every tag and their content are assumed to be self-contained by default. The bolted on functionality we use for functions, method, class- and enum synopses, etc. break that assumption and we have to do some additional workarounds to make nested tags work (as is the case with <constant>s with nested <replacabeable> tags). This is the same additional workaround that needs to be applied to <function> and <methodname> tags with <replaceable> parts so those render properly as well.

[Girgias]

Right, the stream based notion is somewhat annoying.