Add new MIDI Host Class driver to the library, and new MIDIHost ClassDriver demo.

Make MouseHost and KeyboardHost ClassDriver demos use the HID Class driver's structures for the boot protocol Mouse/Keyboard report data, rather than rolling their own.

Demos/Host/ClassDriver/MIDIHost/MIDIHost.txt

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+/** \file
+ *
+ *  This file contains special DoxyGen information for the generation of the main page and other special
+ *  documentation pages. It is not a project source file.
+ */
+ 
+/** \mainpage MIDI Host Demo
+ *
+ *  \section SSec_Compat Demo Compatibility:
+ *
+ *  The following table indicates what microcontrollers are compatible with this demo.
+ *
+ *  - AT90USB1287
+ *  - AT90USB647
+ *
+ *  \section SSec_Info USB Information:
+ *
+ *  The following table gives a rundown of the USB utilization of this demo.
+ *
+ *  <table>
+ *   <tr>
+ *    <td><b>USB Mode:</b></td>
+ *    <td>Host</td>
+ *   </tr>
+ *   <tr>
+ *    <td><b>USB Class:</b></td>
+ *    <td>Audio Class Device</td>
+ *   </tr>
+ *   <tr> 
+ *    <td><b>USB Subclass:</b></td>
+ *    <td>MIDI Subclass</td>
+ *   </tr>
+ *   <tr>
+ *    <td><b>Relevant Standards:</b></td>
+ *    <td>USBIF USB MIDI Audio Class Standard</td>
+ *   </tr>
+ *   <tr>
+ *    <td><b>Usable Speeds:</b></td>
+ *    <td>Full Speed Mode</td>
+ *   </tr>
+ *  </table>
+ *
+ *  \section SSec_Description Project Description: 
+ *
+ *  MIDI host demonstration application. This demo will enumerate an attached USB-MIDI device, and print incomming MIDI note
+ *  on and off messages on any channel to the serial port. Pressing the board joystick will send note on and off messages to
+ *  the attached MIDI device, with the board HWB controlling the note channel.
+ *
+ *  \section SSec_Options Project Options
+ *
+ *  The following defines can be found in this demo, which can control the demo behaviour when defined, or changed in value.
+ *
+ *  <table>
+ *   <tr>
+ *    <td>
+ *     None
+ *    </td>
+ *   </tr>
+ *  </table>
+ */