added default constructors to KeyEvent, TouchEvent, and Touch

In some cases, it's nice to store an Event in your own class for later reference to its various params, which is curently difficult because the above mentioned don't have default constructors.  This adds them (MouseEvent already has one).

In my case I just wanted to store a single Touch (I have multitouch disabled so I want to store the first in the vector), which was also difficult because without this modification, I would have to call TouchEvent::Touch's very long constructor in my class's constructor with a bunch of Vec2f()'s, etc.