Handle wxDefaultSize correctly in wxNonOwnedWindow under wxOSX.

Handle wxDefaultSize correctly in wxNonOwnedWindow under wxOSX.

Creating a wxNonOwnedWindow with default size created tiny, practically
invisible windows because the default size was only taken care of at
wxTopLevelWindow level but not in wxNonOwnedWindow itself. In particular, this
broke creation of wxMDIChildFrames with the default size as this class only
derived from wxNonOwnedWindow and not from wxTopLevelWindow under OS X. It
also probably wasn't intentional as the code did use {Width,Height}Default()
functions but they were wxWindow methods and not the wxTopLevelWindow
(confusingly and error-pronely) named the same.

Fix this, remove the now redundant checks for the default size in
wxTopLevelWindow itself and also rationalize and condense the checks for the
default position and size components in wxNonOwnedWindow::Create().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65261 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775