Use correct expander size in wxDataViewCtrl under MSW

Add wxRendererNative::GetExpanderSize(), implement it using the
appropriate theme element and use it instead of hardcoding the expander
size to 3 character widths.

Closes https://github.com/wxWidgets/wxWidgets/pull/1431

Closes #18449.

interface/wx/renderer.h

@@ -236,6 +236,8 @@ public:
 
     virtual wxSize GetCheckBoxSize(wxWindow *win);
 
+    virtual wxSize GetExpanderSize(wxWindow* win);
+
     virtual void DrawPushButton(wxWindow *win, wxDC& dc,
                                 const wxRect& rect, int flags = 0 );
 
@@ -558,6 +560,16 @@ public:
     */
     virtual wxSize GetCheckBoxSize(wxWindow* win) = 0;
 
+    /**
+        Returns the size of the expander used in tree-like controls.
+
+        @param win A valid, i.e. non-null, window pointer which is used to get
+            the theme defining the expander size under some platforms.
+
+        @since 3.1.3
+     */
+    virtual wxSize GetExpanderSize(wxWindow* win) = 0;
+
     /**
         Returns the height of a header button, either a fixed platform height if
         available, or a generic height based on the @a win window's font.