CX Source and language selector UX enhancements

Followup: I3ad8b50c6bfdc09a33eb77a62c649b5450999319

* Added message documentation
* Change the style of button to mw-ui-progressive(blue)
* Change the button label from Create "translation" to "Start translation"
* Removed the non-functioning close button

Change-Id: I9b66dc691e52eb28a5b9afa0e3ea16a6f928057c