Added new USB_Device_GetFrameNumber() and USB_Host_GetFrameNumber() functions to retrieve the current USB frame number.

Added new USB_Host_EnableSOFEvents(), USB_Host_DisableSOFEvents() and EVENT_USB_Host_StartOfFrame() for the user application handling of USB Start of Frame events while in USB Host mode.

Changed over all demos, drivers and internal functions to use the current frame number over the Start of Frame flag where possible to free up the Start of Frame flag for interrupt use in the user application.