Add an outer border around the textbox popup

[jwlodek]

At the moment, the textbox popup doesn't have any padding or a border around it, which makes it look somewhat awkward on busy TUIs. For example in the podcast player TUI I'm writing, the episode list in the middle of the screen really brings this out:

The two solutions are:

• Add some padding around the popup
• Add an outer border around the popup

Personally, the second option would be more appropriate, but presumably we can offer both as an option.