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.

Deleted Host ClassDriver demos -- not yet started, faster to rebase new ClassDriver demos from the newly schedulerless LowLevel host demos rather than re-convert each demo from the previous scheduler-based implementations.

Fixes to MassStorageHost demo to improve compatibility with more USB drives.

Converted Host mode demos to schedulerless. Fixed host mode broken due to earlier Start-of-frame event experiments.