Adds an export button that outputs selected requirements to a file.
This file is created if it is non-existant.
This file is currently in wpi-suite/Janeway and named requirements.txt.

TODO: Allow the user to input their own file name
TODO: Move the button to a better place
TODO: Add transaction history and acceptance tests to the file output

Manually merge by Long Nguyen Duc Hoang <lhnguyenduc@wpi.edu>
Original codes by benkorza.
Merge pull request #121 from hlong290494/sbl-dev

Adds an export button that outputs selected requirements to a file.