Override Do{Freeze,Thaw}() instead of {Freeze,Thaw}() in wxPropGrid code.

{Freeze,Thaw}() themselves are not virtual any more, so overriding them
doesn't really work and it is unnecessary to reimplement the reference
counting already done by the base class anyhow, so override the DoXXX()
versions instead.

Closes #15877.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75640 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775