widgets to add to Nottui_widgets

[c-cube]

Here are some things I think would be convenient for a lot of UIs. I can contribute some of them if needed :).

I also think that Nottui_widget should just be a module in nottui, there's not much point having one without the other imho.

• basic layout:
  • join_x_list ( reexpose Nottui.Ui.hcat?)
  • pack_x_list (on lists of ui Lwd.t, more convenient than manually using the monoid)
• buttons
  • button (with text and attributes)
  • buttonf (button, + formatter)
  • possibly a family of pre-themed buttons (à la bootstrap: primary, secondary,
    warning, danger, etc. cf https://getbootstrap.com/docs/4.4/utilities/colors/)
  • toggle
• line-wrapped text
• itemized lists (the equivalent of <ul> <li>…</li><li>…)
• grids/tables (with padding options, etc.)
• tree with fold/unfold button at each node (and content lazily loaded)
• progress bar, progress animations (like a \ | / - animation, or something)
• scroll area as big as possible
• modal dialog (about/help/confirmation of action)
• dropdown list selection