Create a plot file that visualises the latency criticality of code
regions.

Really short regions confuse the correlation computations because the
duration is always zero. In this case, we assign the correlation value 0
because such a short duration is not latency-critical at all.

This function is similar to _happens_directly_before, but it should
work better for intra-thread durations.

Remove macros that are unused, confusing, or bugged.

- Make X-Lap disabled by default
- Enable X-Lap if the macro XLAP is defined