Support creation of more than one window.

We always create a single window and assume thats the only window present.
This patch removes this restriction and every GetAcceleratedWidget call will
create a new widget. Now, we also support mapping opaque handle to a widget.
Remove handle generation code in Window.

Now, Display is responisble for generating and mapping handles to widget.
This patch removes similar code in Window class.
Merge pull request #88 from kalyankondapally/MapWindowReview

Map window review