Fix initial wxRadioBox buttons positions in wxMSW.

Fix initial wxRadioBox buttons positions in wxMSW.

We only updated the button positions when the radio box was moved or resized
after being created but didn't do it initially, so a radio box created with
fixed position and size didn't lay out its buttons correctly. Do lay them out
immediately after creating the radio box to fix this.

Closes #13912.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70498 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775