Add ability to take a pin back to the unconfigured state, after chosing another option

[andrewdavidmackenzie]

Is your feature request related to a problem? Please describe.
Selection of Pin functionality from the GUI drop down

Describe the solution you'd like
When a pin has a function, have a "Not used" or "Not configured" option in the drop down, so a used can "un-configure" it

Describe alternatives you've considered
??

Additional context
Might be important when playing with real HW. As you don't necessarily want outputs driven. You could damage hw or have unexpected behaviour.