Upgrade RNX UI to v0.18.0: PropTypes

In v0.18.0, we adapted rnx-ui to react@16.0.0, so we import PropTypes from prop-types instead of react.

If your react version is lower than v15.3.0 and you are not using prop-types, you may meet the warning:

Calling PropTypes validators directly is not supported by the `prop-types` package.
Use PropTypes.checkPropTypes() to call them.

I find no easy way to solve this problem but only use prop-types.

This is the time. 😏