Use identical aliases in multi domain setups

In a multi domain setup it is not possible to use the same alias several time (once for each domain). This extension allows the definition of an alias prefix for each domain, which is only shown internally but not on the web page. 

Example: 
Domain 1: sub.foo.bar, prefix "sub/"
Domain 2: some.foo.bar, prefix "some/"

Page sub.foo.bar/kontakt.html gets alias "sub/kontakt"
Page some.foo.bar/kontakt.html get alias "some/kontakt"