Refactor initialization code in wxTopLevelWindowMSW::Create().

Refactor initialization code in wxTopLevelWindowMSW::Create().

No real changes as the old code was, in fact, correct (although Create()
didn't initialize m_parent explicitly, it was still done in AddChild() if the
parent was not NUL) but just make it more explicit and clear.

Add a new helper wxWindowBase::CreateBase() overload for top level windows,
not taking the validator parameter which doesn't apply to them.

Also make CreateBase() protected as it is only meant to be called from derived
classes Create().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62307 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775