Added ability to customize the handle of a pick_list

[casperstorm]

This PR adds the ability to control the accessory content to the right of pick_list.
Currently in master it is only possible to change the icon size. This PR aims to add a little more customization options:

• Size
• Font
• Content
• Color

This gives the ability to add your own icons/text from your own font packs in various sizes. Example:

let accessory_content = pick_list::AccessoryContent::Custom { 
    font: FONT_ICONS,
    content: String::from('\u{f1a7}'),
    size: Some(14),
};

let pick_list = pick_list(..).accessory_content(accessory_content);

Breaking changes

1. icon_size has been removed from Appearance.
2. accessory_content_color has been added to Appearance.

[hecrj]

Awesome!

I have just renamed AccessoryContent to Handle and the Default variant to Arrow in 39f4918. I think they are more intuitive!

Let me know what you think, and we can merge!

[casperstorm]

Awesome!

I have just renamed AccessoryContent to Handle and the Default variant to Arrow in 39f4918. I think they are more intuitive!

Let me know what you think, and we can merge!

Great names, thanks. LGTM.