No real changes, just extract wxAuiTabArt into its own file. Also rename
wxAuiDefaultTabArt to wxAuiSimpleTabArt in preparation for having other tab
art classes as the default one.
See #14098.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71001 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775