Basic multi-choice selection dialog for the TUI

[krissetto]

Initial version of a multi-choice dialog box.

The idea was to use it to allow for a "reason" dialog box when rejecting tool calls, or to allow the model to prompt users with options via a tool call (not included in this PR).

What does it include?

• Allows defining up to 10 options to choose from
• Choosing if "secondary" button (in the img "skip") is shown along with the primary button, and ability to give them custom labels
• Tab selection between primary/secondary buttons
• If present, secondary button is focused by default, primary gets focused when an option is chosen (e.g. "skip/send")
• Options can be chosen with numbers or by mouse click
• Optional custom input field. If present, typing any non-number will auto select this option and insert the input

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Demo usage screenshot