Use wxTE_RICH style for the log window in the text sample.

Use wxTE_RICH style for the log window in the text sample.

This makes it have nicer colour (white instead of grey by default) under MSW
and wxTE_RICH was already in the code, just commented out -- it seems it's
better to enable it (as it also allows more text to be shown in the control
under old Windows versions) than to remove it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62719 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775