Cleanup of MsgBox

- Expose root container of form to subclasses
- Remove priority from Control. Rendering order is determined when a
  control is added to a Form so this property was pointless.
- Remove build() from Form. Controls should be added to the form before it
  is put into the Stack. If a control is marked not visible while the Form
  is on the stack, the Stack will account for this at a later time.

Signed-off-by: Chris Sanders <zidane@outlook.com>