Close NSWindows properly

Don't just release the window, call the close method. This is usually
the same, but in some situations, the difference (close sends
NSWindowWillCloseNotification, which wx doesn't use) is significant.

In particular, if the window is shown as fullscreen, OS X won't dispose
of its space correctly when the window is released, but not closed
first. See https://github.com/vslavik/poedit/issues/119

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78178 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775