USBMouse relative positioning example

The example emulates mouse over USB port.

1. Flash the board, and ensure the target's auxiliary USB is plugged into the PC.
2. Reset the target.
3. The mouse pointer should move on the screen making a cycle.

Note: With a relative mouse, the USBMouse move function will move the mouse x, y pixels away from its previous position on the screen. MIRRORED FROM MASTER EXAMPLE SNIPPETS REPOSITORY: mbed-os-examples-docs_only. ANY CHANGES MADE DIRECTLY TO THIS REPOSITORY WILL BE AUTOMATICALLY OVERWRITTEN.