Set colours and fonts for all elements of the generic wxSearchCtrl.

Derive generic wxSearchCtrl from wxCompositeWindow<> to ensure that calls to
its Set{Fore,Back}groundColour() are propagated to all its children, otherwise
they applied only to the composite control itself but not e.g. its text part.

Also update the bitmaps when the background colour changes to ensure that it
appears as transparent.

Closes #13428.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68911 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/srchctlg.h

@@ -191,6 +191,7 @@ public:
 
     // wxWindow overrides
     virtual bool SetFont(const wxFont& font);
+    virtual bool SetBackgroundColour(const wxColour& colour);
 
     // search control generic only
     void SetSearchBitmap( const wxBitmap& bitmap );
@@ -235,6 +236,9 @@ protected:
 private:
     friend class wxSearchButton;
 
+    // Implement pure virtual function inherited from wxCompositeWindow.
+    virtual wxWindowList GetCompositeWindowParts() const;
+
 #if wxUSE_MENUS
     void PopupSearchMenu();
 #endif // wxUSE_MENUS