Simplify logic in DRWH for multi window support. Add relevant
documentation for the code.

Instead of making the child windows as root window dispatcher,
now root window always acts as the dispatcher. It forwards mouse
events to any child window which is focussed.

Couple of things are broken still:
1)Sub menus don't work properly for some reason.
2)Focus management between different root windows is broken
  after these changes. I will fix it, while solving the issue
  related to passing window handle.