Implement accessibilityActions

[jonthysell]

Right now, the only action a UIA client can make to a RNW app is Invoke and Toggle (which calls Invoke). The remaining actions we expose to UIA are no-ops:

• Select
• AddToSelection
• RemoveFromSelection
• Expand
• Collapse
• Toggle (re-do)

RN60 adds an accessibilityActions property, which lets you register an event handler to catch these invocations from UIA poking the native side. See docs here: https://facebook.github.io/react-native/docs/accessibility#accessibility-actions

Since it just uses string identifiers, this would be as simple as:

• Storing the registered accessibilityActions event names via DynamicAutomationProperties
• Storing a new AccessibilityActionsEventHandler property similar to AccessibilityInvokeEventHandler but with a name parameter
• Having each UIA action list above check the registered names and if specific ones exist ("toggle", "select", etc) then fire the AccessibilityActionsEventHandler

[harinikmsft]

@jonthysell - please sync with @acoates-ms who is driving the upgrade to RN 0.60 and you can start making these changes in the branch where the upgrade is happening in order to avoid any throw away/conflicting work.

[chrisglein]

With the RN60 upgrade almost wrapped up this should be ready for attention. @licanhua I'm adding it to your queue alongside another block of accessibility work.

[licanhua]

@jonthysell , please help to revisit your proposal. I didn't see any value right now to implement accessibility-actions. Currently dynamicAutomationPeer is applied to View, and adding Toggle, expand action doesn't make sense.