Handle wxEVT_SYS_COLOUR_CHANGED in wxAUI classes

Update the colours used when the system theme changes.

This is especially important to use the colour scheme compatible with
macOS 10.14+ dark mode.

Note that this commit is best viewed with "git diff --color-moved".

Closes https://github.com/wxWidgets/wxWidgets/pull/916

include/wx/aui/tabart.h

@@ -109,6 +109,9 @@ public:
                          wxWindow* wnd,
                          const wxAuiNotebookPageArray& pages,
                          const wxSize& requiredBmpSize) = 0;
+
+    // Provide opportunity for subclasses to recalculate colours
+    virtual void UpdateColoursFromSystem() {}
 };
 
 
@@ -185,6 +188,9 @@ public:
                  const wxAuiNotebookPageArray& pages,
                  const wxSize& requiredBmpSize) wxOVERRIDE;
 
+    // Provide opportunity for subclasses to recalculate colours
+    virtual void UpdateColoursFromSystem() wxOVERRIDE;
+
 protected:
 
     wxFont m_normalFont;