Always establish the EventRacer context of the event target creator

In certain corner cases of an empty document unloading the EventRacer log
wasn't available via |EventRacerContext::getLog()|. Example:

  var w = open("xx.html");
  w.unload = function() {}
Silence a false uninitialized varable warning in Release mode
Limit the scope of the "old" log when committing a provisional load

May not be needed, but ought to be safe.
Log firing an event even of there are no event handlers installed

Not having an event handler may mean a race and this is what we are
trying to discover in the first place.
Keep a serial number (unique ID) in event targets and event listeners
Add a utlity method to EventRacerLog to format and intern a string
Record read/write operations related to handling events
Unclutter the output graph

In debug build, do not show mouse move events with no handler as it creates a
lot of clutter in the visualisation of the graph. Obviously this can hide some
races.