use AddHandler to register pointer handlers so we also get 'handled' events

[mrousal]

Originally only unhandled events were propagated up and pointer events from native components with their own handling where ignored. For example clicks on selectable text were ignored etc so this should fix #564.

[rozele]

Thanks @mrousal! I wonder if we need something a bit more robust to detect when a "handled" event should be re-handled or not. For example, Android and iOS have the concept of the "JS responder" (see https://github.com/facebook/react-native/blob/4f886a29a1234c967deae2354bbc5092e0e6595e/ReactAndroid/src/main/java/com/facebook/react/uimanager/UIManagerModule.java#L572). Currently, we're silently ignoring this part of the UIManager, but I wonder if we could leverage this to make this part more robust.

I'm concerned about the impact on, e.g., ScrollView, which relies on the initial press event being canceled to function properly.

[mrousal]

I agree it would be better, but with current implementation relying on pointer events being routed to root view this seems to be one way how to receive all of them.
I have also tested the app some after the change and didn't notice any performance degradation or other issues.

[rozele]

@mrousal - I'm wondering if this could help remove the hack we have in place to support "swipeable rows" (79b8ae4), or if it will also help fix #1591?

[mrousal]

It might help with first one as it makes sure that all pointer events are processed and can be passed to JS (regardless if some native UIElement marked them as handled), based on description I don't think it is related to #1591 as problem there is Parent == null and not events themselves.

[rozele]

Just double checked, doesn't look like this has any impact on our need for ManipulationMode to implement touch handling inside ScrollViewer.

[rozele]

I also confirmed nothing breaks in RNTester.