fix wxSystemSettings::GetColour() for wxGTK3
closes #15519

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74839 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
fix building with WXWIN_COMPATIBILITY_2_8 == 0
wxPG_EX_DISABLE_TLP_TRACKING does nothing since r61779

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74840 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775