Replace multiple RescaleBitmap() with wxBitmap::Rescale()

Define a single function and use it in both wxArtProvider and (the
generic implementation of) wxSearchCtrl instead of repeating the same
code elsewhere.

Note that another, but slightly different, version of RescaleBitmap()
still remains in wxPropertyGrid.

Deprecate undocumented wxArtProvider::RescaleBitmap() which is
completely useless now.

No real changes, this is just a refactoring.

interface/wx/bitmap.h

@@ -682,6 +682,16 @@ public:
     */
     static bool RemoveHandler(const wxString& name);
 
+    /**
+        Rescale the given bitmap to the requested size.
+
+        This function uses wxImage::Rescale() to resize the given @a bmp to the
+        requested size. Both the bitmap itself and size must be valid.
+
+        @since 3.1.6
+     */
+    static void Rescale(wxBitmap& bmp, const wxSize& sizeNeeded);
+
     /**
         Saves a bitmap in the named file.