wendelin.core: Add extension to activate Wendelin.core 2 preview

component/wendelin.core/activate-WC2-preview.cfg

+# SlapOS extension that switches Wendelin.core to Wendelin.core 2 preview.
+#
+# Should go away once wendelin.core 2 lands to nexedi/wendelin.core@master.
+# Must be extended from last, for example:
+# 
+# [buildout]
+# extends =
+#   .../stack/erp5/buildout.cfg
+#   .../component/wendelin.core/activate-WC2-preview.cfg
+
+[wendelin.core-repository]
+repository = https://lab.nexedi.com/kirr/wendelin.core.git
+branch   = t
+revison  =