(patch #88) displayobject/clone: Fix error with descriptors

The last version broke the clone method in some cases, when the
properties were not directly on the object but further up the
prototype chain. oCanvas now polyfills the planned ES6 method
Object.getPropertyDescriptor to make it look through the
prototype chain as well.
(patch) events: Set correct context for handlers

Event handlers attached to the canvas instance should get the
correct context ('this'). It used to get the canvas element, and
not the core instance.
(patch) mouse & touch: Prevent error in onCanvas method

If no event object was passed to the onCanvas method (or if there
has been no previous mouse/touch event), the updatePos method
should not be called (since it doesn't have any new coordinates).
Update version number to 2.5.1