tools: add "libinput measure touchpad-tap"

tools: add "libinput measure touchpad-tap"

This is the first tool of many more to come to enable users to gather
information aobut their devices and/or usage of these devices. Previously,
these required the users to record events, submit them to a bugzilla, have me
run various scripts over it and then decree that the scripts have spoken.
Push some of this into the hands of the users so they can query the numbers
locally and start investigating (or at least get an idea of what's happening).

This tool measures the time deltas between touch up and touch down and prints
a basic summary, together with the ability to print a dat file with the data
for visualization by e.g. gnuplot. Eventually, more of the current analysis
scripts will be moved into this or other helpers.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>