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/auibar.h

@@ -344,6 +344,10 @@ public:
     virtual int ShowDropDown(
                          wxWindow* wnd,
                          const wxAuiToolBarItemArray& items) = 0;
+
+    // Provide opportunity for subclasses to recalculate colours
+    virtual void UpdateColoursFromSystem() {}
+
 };
 
 
@@ -429,6 +433,8 @@ public:
     virtual int ShowDropDown(wxWindow* wnd,
                              const wxAuiToolBarItemArray& items) wxOVERRIDE;
 
+    virtual void UpdateColoursFromSystem() wxOVERRIDE;
+
 protected:
 
     wxBitmap m_buttonDropDownBmp;
@@ -651,6 +657,7 @@ protected: // handlers
     void OnLeaveWindow(wxMouseEvent& evt);
     void OnCaptureLost(wxMouseCaptureLostEvent& evt);
     void OnSetCursor(wxSetCursorEvent& evt);
+    void OnSysColourChanged(wxSysColourChangedEvent& event);
 
 protected:
 

include/wx/aui/auibook.h

@@ -226,6 +226,7 @@ protected:
     void OnKillFocus(wxFocusEvent& event);
     void OnChar(wxKeyEvent& event);
     void OnCaptureLost(wxMouseCaptureLostEvent& evt);
+    void OnSysColourChanged(wxSysColourChangedEvent& event);
 
 protected:
 
@@ -408,6 +409,7 @@ protected:
     void OnTabRightUp(wxAuiNotebookEvent& evt);
     void OnTabBgDClick(wxAuiNotebookEvent& evt);
     void OnNavigationKeyNotebook(wxNavigationKeyEvent& event);
+    void OnSysColourChanged(wxSysColourChangedEvent& event);
 
     // set selection to the given window (which must be non-NULL and be one of
     // our pages, otherwise an assert is raised)

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);

include/wx/aui/framemanager.h

@@ -616,6 +616,7 @@ protected:
     void OnChildFocus(wxChildFocusEvent& evt);
     void OnHintFadeTimer(wxTimerEvent& evt);
     void OnFindManager(wxAuiManagerEvent& evt);
+    void OnSysColourChanged(wxSysColourChangedEvent& event);
 
 protected:
 

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;