Added ComboBox widget.

[jonathanslenders]

If we want this in prompt_toolkit itself, this is one way to do it.

• Maybe this should be build on top of the RadioList instead, with an option to hide the radios, and show the background-color.
• Maybe we shouldn't merge this, because it doesn't belong in prompt_toolkit itself.
• Maybe it should not be called ComboBox.

TODO: add the style. Something like:

    {
        "dialog.body select-box": "bg:#cccccc",
        "dialog.body select-box cursor-line": "nounderline bg:ansired fg:ansiwhite",
        "dialog.body select-box last-line": "underline",
    }