Add wxAuiTabArt::SetColour() and SetActiveColour() methods and provide trivial
default implementation of them in wxAuiDefaultTabArt to allow customizing the
tab colours.
Closes#11411.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67428 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775