gdk/Win32: Fix mouse pointer capture

gdk/Win32: Fix mouse pointer capture

Call SetCapture() explcitly for the (new) modal window so that we make the
modal window respond to mouse input, and also call SetCapture() to the parent
of the transient window that we are destroying so that mouse input capture is
returned to the parent window.

This attempts to fix the following:

*  Upon creating a new modal window, the new modal window does not receive
   pointer input unless one switches to another program and back

*  Upon closing a transient window, the parent window that activated the
   transient window does not receive pointer input unless one switches to
   another and back