Added Debian version to info box and point to Debian website

I think it makes sense to have an possibility to define a specific vender version, domain and url in the info box. I created three different parameters, 'cause it is unclear, if APPLICATION_DOMAIN is allowd to change. Btw. APPLICATION_VENDOR is not used in the owncloudtheme.

this patch is not ready to merge :)

For those, who ask DEBIAN_VERSION is setted via direct cmake parameter.