Add MIDI class driver, update MIDI device demo to use the new USB class driver. All device demos/projects have now been converted to the new library class drivers, although documentation is yet to be completed.

Commit of new class abstraction APIs for all device demos other than the MIDI demo - not documented yet.

Removed scheduler and memory allocation libraries.

Added new EVENT_USB_StartOfFrame event in the library to indicate the start of each USB frame (when generated).

Removed Tx interrupt from the USBtoSerial demo; now sends characters via polling to ensure more time for the Rx interrupt.