This PR adds support to close the current wallet in the GUI.
<img width="543" alt="screenshot 2019-01-18 at 00 44 26" src="https://user-images.githubusercontent.com/3534524/51358241-424b9680-1aba-11e9-88f2-b85869507737.png"> <img width="532" alt="screenshot 2019-01-18 at 00 44 38" src="https://user-images.githubusercontent.com/3534524/51358242-424b9680-1aba-11e9-83e2-fa275a9017b3.png">