Overhaul TextInput's API.

Closes #10 and closes #18.

This change is backward compatible. However, it deprecates the use of Text, preferring Value instead! This change adds the suggested API from #18 to TextInput instead of a separate component.

I'm not sure if OnFocusLostInvalid is required since OnChangedInvalid exists. However, text has been passed to OnFocusLost previously which now could be invalid.

In the future, Text should be completely removed in a future major release.

This change allow us to implement #9 or any other formatted text input with ease as seen in the example story.