Use more appropriate colours for AUI dock art under Mac

Use system colours instead of hard-coding white.

This is still not ideal because ideally the same wxSYS_COLOUR_XXX
constants should be usable under all platforms, but better than just
using completely wrong colours when using dark mode under Mac.

src/aui/dockart.cpp

@@ -268,8 +268,8 @@ wxAuiDefaultDockArt::InitBitmaps ()
         0x7f,0xff,0xff,0xff,0xff,0xff,0xff,0xff};
 
 #ifdef __WXMAC__
-    const wxColour inactive = *wxWHITE;
-    const wxColour active = *wxWHITE;
+    const wxColour inactive = wxSystemSettings::GetColour(wxSYS_COLOUR_INACTIVECAPTION);
+    const wxColour active = wxSystemSettings::GetColour(wxSYS_COLOUR_CAPTIONTEXT);
 #else
     const wxColor inactive = m_inactiveCaptionTextColour;
     const wxColor active = m_activeCaptionTextColour;