v0.3.0-beta.0

Added

• Add a basic Crossterm backend

Changed

• Remove Group and introduce Layout in its place
  • Terminal is no longer required to compute a layout
  • Size has been renamed Constraint
• Widgets are rendered on a Frame instead of a Terminal in order to
  avoid mixing draw and render calls
• draw on Terminal expects a closure where the UI is built by rendering
  widgets on the given Frame
• Update Widget trait
  • draw takes area by value
  • render takes a Frame instead of a Terminal
• All widgets use the consumable builder pattern
• SelectableList can have no selected item and the highlight symbol is hidden
  in this case
• Remove markup langage inside Paragraph. Paragraph now expects an iterator
  of Text items