This is done to fix the build error in STL builds after the previous
commit, as using Find() or DeleteObject() with a pointer of the "base
type" worked when using wxWindowList in the default build, but not in
the STL build, so this commit avoids this mismatch by allowing to use a
base class pointer where it is safe to do so, i.e. where it can be done
without a downcast, as there doesn't seem to be any reason to have this
difference between the builds in this case.
Note that there is still a difference between Append() and Insert()
methods which (correctly) require the pointer of right type in STL build
but still accept a pointer of the base type in the default build for
compatibility reasons.
In particular, this means that one of the casts removed in the last
commit from wxWindowBase code needs to be restored, as Append() still
requires a wxWindow and not just a wxWindowBase.
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