Make background of book controls in wxMSW be the same as parent

This improves their appearance especially when they're used as children of
wxNotebook, which uses a different background colour than plain wxPanel by
default.

This generalizes what was already done for wxChoicebook in 6cab632f
(see #12503) to all book controls.

Closes #16878.

include/wx/bookctrl.h

@@ -311,6 +311,11 @@ protected:
     // Lay out controls
     virtual void DoSize();
 
+    // It is better to make this control transparent so that by default the controls on
+    // its pages are on the same colour background as the rest of the window. If the user
+    // prefers a coloured background they can set the background colour on the page panel
+    virtual bool HasTransparentBackground() wxOVERRIDE { return true; }
+
     // This method also invalidates the size of the controller and should be
     // called instead of just InvalidateBestSize() whenever pages are added or
     // removed as this also affects the controller

include/wx/choicebk.h

@@ -79,11 +79,6 @@ public:
     // returns the choice control
     wxChoice* GetChoiceCtrl() const { return (wxChoice*)m_bookctrl; }
 
-    // Override this to return true because the part of parent window
-    // background between our controlling wxChoice and the page area should
-    // show through.
-    virtual bool HasTransparentBackground() wxOVERRIDE { return true; }
-
 protected:
     virtual void DoSetWindowVariant(wxWindowVariant variant) wxOVERRIDE;