Password manager security: Remove current HEAD item when new empty string is copied

[jasonm23]

Feature description

Password managers will often delete (clear) the clipboard n seconds after copying a password, as a security measure.

We can remove current HEAD item when new empty string is copied (i.e. delete (clear) clipboard)

Simple implementation, detect empty -> delete head/current history.

[jasonm23]

Tests pass. However, how various password managers clear the clipboard is TBD.