erp5_web_widget_library: Add a static placeholder browserconfig.xml file.

Reduces the number of meaningless 404 errors.

bt5/erp5_web/SkinTemplateItem/portal_skins/erp5_web_widget_library/browserconfig.xml._xml

+<?xml version="1.0" encoding="utf-8"?>
+<browserconfig>
+    <msapplication>
+    </msapplication>
+</browserconfig>

bt5/erp5_web/SkinTemplateItem/portal_skins/erp5_web_widget_library/browserconfig.xml.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="File" module="OFS.Image"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>_EtagSupport__etag</string> </key>
+            <value> <string>ts74276931.87</string> </value>
+        </item>
+        <item>
+            <key> <string>__name__</string> </key>
+            <value> <string>browserconfig.xml</string> </value>
+        </item>
+        <item>
+            <key> <string>content_type</string> </key>
+            <value> <string>text/xml</string> </value>
+        </item>
+        <item>
+            <key> <string>precondition</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>size</string> </key>
+            <value> <int>113</int> </value>
+        </item>
+        <item>
+            <key> <string>title</string> </key>
+            <value> <string></string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>