Preserve the button attributes when setting its bitmap in wxGTK

The button label font and colour could be lost when its bitmap was set because
this could result in the recreation of the GtkLabel, internally.

Fix this by reapplying the styles to the possibly recreated label when
changing the bitmap or its position in the button.

src/gtk/anybutton.cpp

@@ -230,6 +230,11 @@ void wxAnyButton::DoSetBitmap(const wxBitmap& bitmap, State which)
                 {
                     image = gtk_image_new();
                     gtk_button_set_image(GTK_BUTTON(m_widget), image);
+
+                    // Setting the image recreates the label, so we need to
+                    // reapply the styles to it to preserve the existing text
+                    // font and colour if they're different from defaults.
+                    GTKApplyWidgetStyle();
                 }
                 else // image presence or absence didn't change
                 {
@@ -411,6 +416,11 @@ void wxAnyButton::DoSetBitmapPosition(wxDirection dir)
         }
 
         gtk_button_set_image_position(GTK_BUTTON(m_widget), gtkpos);
+
+        // As in DoSetBitmap() above, the above call can invalidate the label
+        // style, so reapply it to preserve its font and colour.
+        GTKApplyWidgetStyle();
+
         InvalidateBestSize();
     }
 #endif // GTK+ 2.10+