Add wxDPIChangedEvent::Scale() and use it in this event handlers

This is more concise and less error-prone than multiplying/dividing DPI
values manually.

No real changes except, maybe, in wxSlider code where the rounding was
done differently before for some reason.

interface/wx/event.h

@@ -3444,6 +3444,38 @@ public:
         Returns the new DPI.
     */
     wxSize GetNewDPI() const;
+
+    /**
+        Rescale a value in pixels to match the new DPI.
+
+        This is a convenience function to use in wxEVT_DPI_CHANGED event
+        handlers, as they often need to update some sizes to the new DPI.
+        It simply calls wxRescaleCoord() with GetNewDPI() and GetOldDPI().
+
+        For example, the returned value will be twice bigger than the original
+        one when switching from normal (96) DPI to high (2x, 192) DPI.
+
+        @since 3.1.6
+     */
+    wxSize Scale(wxSize sz) const;
+
+    /**
+        Rescale horizontal component to match the new DPI.
+
+        This is the same as Scale(), but for the horizontal component only.
+
+        @since 3.1.6
+     */
+    int ScaleX(int x) const;
+
+    /**
+        Rescale vertical component to match the new DPI.
+
+        This is the same as Scale(), but for the vertical component only.
+
+        @since 3.1.6
+     */
+    int ScaleY(int y) const;
 };