Add a new PlProfiler tool based on PlBacktrace.

Add a new PlProfiler tool based on PlBacktrace.

To use:
Just add
PlProfiler::start("category", "event");
PlProfiler::stop("category");

so, if you want both to profile a loop and each step of the loop:
PlProfiler::start("my loop");
for ($i = 0 ; $i < 2000 ; ++$i) {
  PlProfiler::start("my step", $i);
  do_sth();
  PlProfiler::stop("my step");
}
PlProfiler::stop("my loop");

The result will be displayed as a backtrace by category.

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>