Improve issue display in tool window

[david-yz-liu]

1. Adding Different Icons for Different Issues: Currently, all the issues identified by PyTA with a file are displayed in the same text style. One way in which this can be changed is adding different icons for different issues, based on the severity of the issue (something similar to what is done with the Pylint Plugin).
2. Adding a “Navigate to Line” Feature in the Tool Window: One nice feature that can be added to the Tool Window is automatic navigation to the line which contains the issue (according to PyTA) when the user clicks on a particular issue in the Tool Window.