Merge dom and component focus (#978)

Instead of two levels of focus with `focus` and `selected`, use a recursive
level. The components set the one "active" and hbox/vbox/dbox 

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

CHANGELOG.md

@@ -61,6 +61,8 @@ current (development)
   - See `selectionForegroundColor` decorator.
   - See `selectionStyle(style)` decorator.
   - See `selectionStyleReset` decorator.
+- Breaking change: Change how "focus"/"select" are handled. This fixes the
+  behavior.
 
 ### Screen
 - Feature: Add `Box::IsEmpty()`.