Requests for comment/Statement group ordering

=RFC Property Sorting=

The idea of this RFC is to figure a way to sort properties- especially for the Article Placeholder extension but with the idea in mind to be able to use the sorted list also for other extensions/purposes.

Requirements
The list for sorting properties should
 * have a sane default (maintained on wikidata)
 * be editable by the local Wiki communities independently
 * and therefore be stored and editable on the Wiki

Implementation details
The idea is to have a individual Wikipage to store the list on the different Wikiprojects, so it's accessible to anyone and can be edited and maintained according to the needs of the communities. This page should probably be in the mediawiki namespace- in this way it's also only editable by admins.

The list should contain of a line per entity with additional comments and probably also the label of the item. To parse the list, we should think if we either want to have the list in Wikitext or as a new content model (text based, for now). Either way, the edit page will just be normal plain text, so it's easier to sort also bigger chunks of properties. We will have to parse the relevant info (ordered list of property ids) from irrelevant (comments, sections, documentation). The list should probably in the MediaWiki namespace and be identified simply by name. But that will possibly mean that we also have just one list per Wiki. If we want to allow multiple lists and/or put the list under an arbitrary title and namespace, we need a mechanism for creating a wiki page with the appropriate content model.

We will need to look into caching of the list. We will probably go for a shared object cache.