Avoid asserts in wxGTK when wxMDIChildFrame::SetSize() is called.

SetSize() doesn't do anything for MDI children in wxGTK and other ports using
TDI version of MDI, but it shouldn't result in an assert from
wxTLW::DoMoveWindow() neither, so override DoMoveWindow() in wxTDIChildFrame
to avoid it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75523 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775