Fix showing MDI child from wxDocManager::ActivateDocument().

Fix showing MDI child from wxDocManager::ActivateDocument().

This fix has 2 components: first, call Raise() instead of simple SetFocus()
from ActivateDocument() as this corresponds better to what we want to do here.
And second, override Raise() to call Activate() in wxMDIChildFrame to ensure
that it works as expected as raising an MDI child doesn't work otherwise in
most ports (with the exceptions for wxOSX which, on the contrary, needs a
small fix to preserve the existing working Raise() behaviour).

Closes #13946.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70790 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775