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/dockart.h

@@ -76,6 +76,9 @@ public:
                           int buttonState,
                           const wxRect& rect,
                           wxAuiPaneInfo& pane) = 0;
+
+    // Provide opportunity for subclasses to recalculate colours
+    virtual void UpdateColoursFromSystem() {}
 };
 
 
@@ -136,6 +139,9 @@ public:
                   wxAuiPaneInfo& pane);
 #endif
 
+    virtual void UpdateColoursFromSystem() wxOVERRIDE;
+
+
 protected:
 
     void DrawCaptionBackground(wxDC& dc, const wxRect& rect, bool active);