Put normal space between Back and Next buttons in wxWizard.

Put normal space between Back and Next buttons in wxWizard.

Use regular space between these two buttons even on Windows, where they
were previously close together. This used to be done in older Windows
versions, but modern Windows HIG documents show wizards with equal
spacing between all buttons and that's what Windows OS itself does. In
other words, not treating this pair of buttons specially is more native.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66985 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775