Fix redefinition warning when USB_DEVICE_ONLY is set on an AVR which does not support USB host mode.

Added parameter directions to function parameter documentation.

Added new experimental FAST_STREAM_FUNCTIONS compile time option to speed up stream transfers at the expense of a higher FLASH consumption (needs testing to verify improved throughput).

Break device mode class driver interfaces into seperate config and state structs which are then combined, for clarity. Move device mode class driver interfaces back into the device mode class driver headers from the common class headers to make room for host class interfaces.

Change MIDI demos to use real MIDI command values, and shift for the USB wrapper, rather than shift for the MIDI bytes. This is a little confusing for the MIDI USB wrapper, but allows for the use of real standardized MIDI command values.

Remove ConfigDescriptor.c/.h files from the ClassDriver Host demos, as they will be obsoleted when the Host mode class drivers are complete. Add new StillImage stub class driver common header.

Split out common defines/types from class drivers into a seperate common class driver directory.

Make central USB Class driver dispatch headers, used for both device and host modes.