Restore wxPreviewFrame::Initialize(void) and add InitializeWithModality().

Restore wxPreviewFrame::Initialize(void) and add InitializeWithModality().

The changes of r67619 changed the signature of the virtual Initialize() method
and in doing so broke the existing code overriding it. Avoid this problem by
restoring the old method signature and adding a function with a different name
providing the new functionality.

Also notice in the documentation that there is no real need to override
Initialize() in any case (but this doesn't change the fact that there is
existing code that does do it).

See #13108.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68026 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775