Improve ergonomics of overlays / popovers / modals

[brentvatne]

• People want to conceptually just open a modal on a stack without creating a parent stack: https://github.com/react-navigation/react-navigation/pull/2308
• We can't customize transitions on a screen-by-screen basis (only on an entire stack), so we can't do this: https://github.com/react-navigation/rfcs/issues/10
• Creating a Modal stack can be confusing to people: https://github.com/react-navigation/react-navigation/issues/3447
• Modals can be full screen and opaque (blocking contents behind them), full screen and translucent (eg: window in the middle, rest is a translucent underlay), or a popover that overlays the header or tabbar, or other kinds of partial overlays. https://github.com/react-navigation/react-navigation.github.io/issues/7
• People want to be able to pick what direction the modal transitions in from: https://github.com/react-navigation/react-navigation/pull/590
• The React Native built-in Modal component doesn't hook in to navigation events / isn't "navigation-aware" so people run into issues when trying to navigate away from it: https://github.com/react-navigation/react-navigation/issues/3463
• Transparent modal background not working: https://github.com/react-navigation/react-navigation/issues/3630