Shredder doesn't stop rmlint or du on close and doesn't impose a limit

[tbertels]

When we open Shredder, it starts du to find out and display the space usage of each location. If we then start scanning a location, Shredder starts rmlint while keeping du working. This is especially bad for a hard disk. If we click on the "refresh" button on bottom left, Shredder starts an additional rmlint process, while keeping the previously running process active. We can repeat this any number of times to start additional rmlint processes. If we then close Shredder, it will keep the previously started processed running uselessly, so we have to manually close them in a task manager.

Shredder should:

• impose a limit of one running process by device
• close all running processes on exit