simplifications and corrections to background drawing:

1. removed ApplyParentThemeBackground() not used any longer
2. removed ProvidesBackground() which is synonymous with
   !HasTransparentBackground()
3. removed a whole bunch of unused MSWXXX() methods
4. moved MSWControlColor() from wxWindow up to wxControl

results:

1. the gradient is still shown properly for static/radio boxes in notebooks
2. correct background colour is used for the static boxes
3. code is shorter and better commented


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@33474 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/slider.h

@@ -80,9 +80,6 @@ public:
     virtual int GetSelStart() const { return GetMax(); }
     virtual void SetSelection(int WXUNUSED(min), int WXUNUSED(max)) { }
 
-    virtual void ApplyParentThemeBackground(const wxColour& bg)
-        { SetBackgroundColour(bg); }
-
 protected:
 
     // adjust value according to wxSL_INVERSE style