Update AUI bitmaps when its colours change.

Extract the bitmaps initialization in wxAuiDefaultDockArt::InitBitmaps() and
call it from SetColour() to ensure that the bitmaps are updated when the
colour scheme changes.

Closes #12532.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66313 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/aui/dockart.h

@@ -134,6 +134,8 @@ protected:
 
     void DrawCaptionBackground(wxDC& dc, const wxRect& rect, bool active);
 
+    void InitBitmaps();
+
 protected:
 
     wxPen m_border_pen;