Add new option to show clipboard updates in verbose mode

[ssailly]

This implements the feature suggested by #3475.

When copying from the device, then copy-pasting from the computer to the device using Ctrl+V, and running with -Vdebug --show-clipboard :

• Before:

INFO: Device clipboard copied
DEBUG: Computer clipboard set

[server] INFO: Device clipboard set
DEBUG: Computer clipboard unchanged

• After:

INFO: Device clipboard copied: This application mirrors Android devices
DEBUG: Computer clipboard set: This application mirrors Android devices

[server] INFO: Device clipboard set: It works on Linux
DEBUG: Computer clipboard unchanged

Running without --show-clipboard has the same behavior as "Before".

[rom1v]

Thank you for your contribution.

Small remarks inline after a quick review.

[rom1v]

By default, the clipboard content must not be printed (it may contain sensitive information).

However, I'm wondering if we could just log it in verbose mode (without adding yet another very-specific niche option). (not sure)