as special events.

If we store all received events to a file, this dump is now
self-contained. Helps to analyze traces.

Some functions have also been factorized in the process, too much
duplication.

(Also tracer/Makefile was forgotten in previous commit, included
in this one...)

many assumptions about X backed were made, to be refined if does not
work properly in some setups

it seems more natural to stop autoscroll with button 3 and start it
with button 1

not very clean, might be reworked at some point
the idea is to compute much less 'log' than what
was done

not very efficient, to be refined

requires those optimizations

is zero in get_dimensions for xy_plot