Add @footnote and @footnoteref doc-comment tags

Especially when javadoc is once again used to generate hardcopy documentation (such as -doctype MIF), it would be useful to add doc-comment tags that translate into the appropriate target code footnotes and references to footnotes. The syntax would look something like:

{@footref identifier} In-line tag
@footnote identifier text Standalone tag

Use the in-line tag in the body of the text, and the standalone tag
later in the normal tag section of the doc comment. The "identifier"
connects "footref" to "footnote".

For example:

/**
* An instance of a scroll bar{@footref scrollbar} is ...
* ...
* @footnote scrollbar A scroll bar is a visual control.
*/

would generate FramkeMaker output such as (where the "1" is superscripted):

An instance of a scroll bar1 is ... (text)
...
1 A scroll bar is a visual control. (footnote)

and would generate HTML output such as:

An instance of a <a href="scrollbar">scroll bar</a> is ...
...
<a name="scrollbar">¹</a> A scroll bar is a visual control.