This prevents stuck keys when switching endpoints by clearing
everything in the HID report and sending one last report before
switching to the new endpoint.
Added some utility functions and an event for tracking the state of the
USB connection.
Updated endpoints.c to select a single endpoint to send HID reports to
based on the status of the USB and BLE connections. Partially fixes#206.
Future commits will add a user setting to control which endpoint is used if
both USB and BLE are ready.
