Some more work on the Doxygen version of the haiku_book.

I will keep working on the Docbook version, but as I'm still not completely comfortable with that as final choice, I'm playing around with this as well.

The String.cpp file contained doxygen documentation for almost all methods, I copied those to a new file (string.dox) and grouped them accordingly. (Done because Axel is absolutely against in-header or in-source docs)

Integrated the BMidiConsumer and BMidiLocalConsumer class into the same file (like I did now to the producers), since I think it's better to keep a 1:1 relation with the headers. 

Removed the mididefs.dox file and replaced it with the midi2/Midi2Defs.dox file which actually documents the Midi2Defs.h file, rather than contain a custom page that was somewhat hard to find.

Please see http://www.myhouserules.nl/haiku_book/index.html for a generated book from the current source. I actually quite like the output so far, though I'm aware of the fact that I needed to perform some tricks to let Doxygen get to this point.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@19651 a95241bf-73f2-0310-859d-f6bbb57e9c96

docs/user/midi2/midiroster.dox

@@ -257,7 +257,7 @@ void MyView::MessageReceived(BMessage* msg)
 }
 \endcode
 
-\sa \ref bmidiop "The possible be:op codes (BMidiOp)"
+For the possible midi options, see #BMidiOp
 */
 
 /*!
@@ -292,3 +292,12 @@ There is no real reason use this function, since all BMidiRoster's public
 function are static.
 
 */
+
+/*!
+\enum BMidiOp
+\brief Defines the status codes for MIDI Server notification messages.
+
+These codes are used when you request notification as in BMidiRoster::StartWatching().
+Check against these codes to determine what is happening. See the StartWatching() method
+for a more complete description of the codes and their meaning.
+*/