Fix wxBitmapButon used as wxPG editor button under wxGTK2

wxBitmapButton without inner border should work better with themes
if we reduce inner border in a more GTK-compliant way by applying
a dedicated GTK style to the button.

src/propgrid/editors.cpp

@@ -2162,28 +2162,36 @@ public:
 #if defined(__WXGTK3__)
         GTKApplyCssStyle("*{ padding:0 }");
 #else
-#if !defined( __WXGTK127__ )
-        GTKApplyWidgetStyle(true); // To enforce call to DoApplyWidgetStyle()
-#endif
+        // Define a special button style without inner border
+        // if it's not yet done.
+        if ( !m_exactFitStyleDefined )
+        {
+            gtk_rc_parse_string(
+              "style \"wxPGEditorBitmapButton_style\"\n"
+              "{ GtkButton::inner-border = { 0, 0, 0, 0 } }\n"
+              "widget \"*wxPGEditorBitmapButton*\" style \"wxPGEditorBitmapButton_style\"\n"
+            );
+            m_exactFitStyleDefined = true;
+        }
+
+        // Assign the button to the GTK style without inner border.
+        gtk_widget_set_name(m_widget, "wxPGEditorBitmapButton");
 #endif
     }
 
     virtual ~wxPGEditorBitmapButton() { }
 
-protected:
-    virtual void DoApplyWidgetStyle(GtkRcStyle *style) wxOVERRIDE
-    {
-        if ( style )
-        {
-#if !defined( __WXGTK127__ )
-            style->xthickness = 0;
-            style->ythickness = 0;
-#endif
-        }
-        wxBitmapButton::DoApplyWidgetStyle(style);
-    }
+private:
+#ifndef __WXGTK3__
+    // To mark if special GTK style was already defined.
+    static bool m_exactFitStyleDefined;
+#endif // !__WXGTK3__
 };
 
+#ifndef __WXGTK3__
+bool wxPGEditorBitmapButton::m_exactFitStyleDefined = false;
+#endif // !__WXGTK3__
+
 #else // !__WXGTK__
 
 typedef wxBitmapButton wxPGEditorBitmapButton;