v0.2.0: Box renderable — border, background, and title

- Box class with border-style, title, fg/bg slots
- render-box dispatches through backend protocol
- draw-border for borders, draw-rect for background
- draw-text for title below top border
- 7 tests: defaults, border, background, title, no-border,
  zero-size, minimum-size
- 13 assertions, 100% GREEN
- ASDF updated with src/components module
- modern-backend now accepts :output-stream initarg

src/components/package.lisp

@@ -0,0 +1,9 @@
+(defpackage :cl-tui.box
+  (:use :cl :cl-tui.backend :cl-tui.layout)
+  (:export
+   #:box #:make-box
+   #:box-layout-node
+   #:box-border-style #:box-title #:box-title-align
+   #:box-fg #:box-bg
+   #:render-box))
+(in-package :cl-tui.box)