Fixed: CPWindow keeps a reference of its firstResponder when closing a window

Previously, when closing a CPWindow, the window kept a reference of the firstResponder. This things raised an issue when opening a window again and asking the window to make the first responder to the same element as the one we had.

Now when closing a window, the window will update its firstResponder to nil.
Merge pull request #2229 from Dogild/WindowCloseFirstResponder

Fixed: CPWindow keeps a reference of its firstResponder when closing a window