UI supports modifying NI

[LauraAubin]

Now that we have implemented our basic UI in #5, we have visualized a list of processes along with their PID and NI value.

The next step is to create buttons that will allow us to increase and decrease the selected process' NI value.

• Create a button that prints to the console Create a view controller with a button that prints to the console #14
• Create buttons that increase/decrease NI value of the selected process Create a button that increases the selected priority #17 and Create button and functions for decreasing NI #18

Completed: