indicate which attribute to use as label for an item

[pvgenuchten]

This is a proposal for optimizing #133, it also brings in an aspect of #100. I wanted to combine the two to see the full benefit of it. In config you can assign one attribute as being the label/name/title of the item.

This value is then used as label on the map and title of the item page

I also introduced the capability to set this value in a multilingual way

In case you ask the french representation of the page, the french attribute will be used as label

This required a language to be taken from accept-language or url-override, which had quite impact on the code, i can image people may want to refactor that aspect, please make suggestions

I considered also an approach in which i would add this aspect as part of the context, maybe @alpha-beta-soup can provide some suggestion there, maybe both approaches can co-exist?

[pvgenuchten]

the language mechanism introduced in #470 seems defined in a more generic way, does it include the option to get language from header. maybe better to merge 470 first and adopt the approach?