Extended pick_list::Handle to be more flexible

[casperstorm]

I have extended the pick_list::Handle slightly:

{
    Arrow,
    Static({ .. }),
    Dynamic {
        closed: { .. },
        open: { .. },
    },
    None,
}

This was done because I felt the previous solution was too limited by not allowing the user to choose different content depending on the state of the pick_list. The main use case is to display a different icon, like ▲ and ▼, depending on the open/close state.

Below is a small gif show casing the different states.

[hecrj]

Looks great! Thanks.

I have simplified some stuff here and there:

• Renamed HandleContent to Icon and made the text be a code_point instead.
• Changed the generic type to be the Font type directly.
• Inlined the Handle::content helper since it's only used in draw once.

Let's merge! 🚢