Major UI revamp.

Changes include:
- Seperated pull request details/comments. They
  are now displayed in two seperate tabs.
- Moved related MVC parts into their own modules
- Properly exit application on close
- Alternate row colors.
- Disable selection on comment tree view.
- Only scroll the contents of the tab, not the tabs as well.
- Fetch comments asynchronusly preventing delays b/w pull
  request selections.
- Show loading icon when loading data