Allow importing failed tests as failure causes

Here is the code to import failed tests as failure causes, as discussed via ticket https://issues.jenkins-ci.org/browse/JENKINS-25115

I split the actual code from the configuration part for clarity. Feel free to squash them together if you prefer.

One note regarding the configuration page: I could not find a way to fit the configuration checkbox+textbox visually in the same way as the existing options. With the current implementation, the checkbox is in the leftmost column with the label on the right instead of in the middle with the label on the left. One could shift the entire block to the right by using f:nested instead of f:block, but still the label would be on the right of the checkbox and the left column would be empty.
Sorry but I can't make this work properly: if I put f:description inside the optionalBlock, then the description is only visible when the checkbox is checked. Placing it above the optionalBlock makes it seem to belong to the previous configuration item.

I think we have three options now:
1. keep the layout as proposed originally. It's not ideal since the checkbox is not placed in the same way as the other entries, but it's the best I've been able to create in my experiments.
2. Stop using optionalBlock, thus always showing the categories input box. This will make it possible to match the layout of the other entries.
3. You or @TWestling finding a better way to display using the optionalBlock.

What is your opinion?