This PR does fix the issue where the keyboard is not shown on the screen when you tap on a AztecWrapper field in RN applications.
It also add 2 other new features (one of those were already present but never exposed to JS) for focus/blur handling in JS.

Details:

• Adds the ability to listen on blur and focus events from the JS side. We can now use onBlur, and onFocus and attach a JS function to it.

• Does fix an issue where the keyboard is not shown on the screen when the edit field get the focus.

• Adds the ability to set focus or unset it via JS. There are 2 new commands (focusTextInput and blurTextInput ) that can be invoked JS side to trigger the native side to get or lost the focus.

Test 1

• Start the demo app
• Check that onBlur and onFocus log lines are present in the log when you jump between fields

Test 2 (External test with a RN app)

• Use gb-mobile master and point react-native-aztec to this hash 357a9a4
• Then start the demo gb-mobile app, and tap/jump between AztecWrapper powered blocks
• The keyboard should be visible on the screen
• Dismiss the keyboard
• Tap on a AztecWrapper powered block. The keyboard should be visible now.

I will provide a GB-mobile side PR once this is merged in master