Change default to not include the icon file to the file thumbnail on save.

Windows renders the application image on top so it appears twice.
Update example file thumbnails on start page to have a consistent background (classic theme) and remove the old FreeCAD logos.

src/Gui/PreferencePages/DlgSettingsDocument.ui

@@ -411,7 +411,7 @@ Common sizes are 128, 256 and 512</string>
          <string>Add the program logo to the generated thumbnail</string>
         </property>
         <property name="checked">
-         <bool>true</bool>
+         <bool>false</bool>
         </property>
         <property name="prefEntry" stdset="0">
          <cstring>AddThumbnailLogo</cstring>

src/Gui/Thumbnail.cpp

@@ -100,7 +100,7 @@ void Thumbnail::SaveDocFile (Base::Writer &writer) const
     if (!img.isNull()) {
         // Create a small "Fc" Application icon in the bottom right of the thumbnail
         if (App::GetApplication().GetParameterGroupByPath
-            ("User parameter:BaseApp/Preferences/Document")->GetBool("AddThumbnailLogo",true)) {
+            ("User parameter:BaseApp/Preferences/Document")->GetBool("AddThumbnailLogo",false)) {
             // only scale app icon if an offscreen image could be created
             appIcon = appIcon.scaled(this->size / 4, this->size /4, Qt::KeepAspectRatio, Qt::SmoothTransformation);
             px = BitmapFactory().merge(QPixmap::fromImage(img), appIcon, BitmapFactoryInst::BottomRight);