VoiceOver Support + Direct Embedding

Some much-needed additions and fixes. This is a non-breaking change.

• Presentation changes.
  • Popovers no longer places an intermediary view controller at the top of the screen.
  • Instead, popovers are directly embedded as subviews of the base UIWindow.
  • This minimizes interference with built-in presentation APIs.
• Software should be accessible. Popovers now comes with built-in VoiceOver support!
  • A dismiss button can be automatically added when VoiceOver is on.
  • The button is fully customizable, along with other traits, via attributes.accessibility.
• General improvements.
  • Frame recalculations after a screen rotation are now much smoother.
  • Popovers no longer steal focus from active controls like text fields.
  • Sheets and popovers can be presented at the same time.

Thanks again to @ShezHsky for the contributions!