Rename UI thread

[Almouro]

In the list of threads (for instance, in this report, we always display the UI thread with the name read in the /proc/... files. But it's usually quite ugly, it's a string that includes all or part of the bundle id.

Furthermore if we want to compare UI thread usage for multiple apps not sharing the same bundle id, it's not so easy to do so. It would be better to identify it and rename it to "UI thread"