Allow ordering the same level pages in the pages tree

[sembrestels]

To do this we'll need to introduce the "priority" metadata in the page subtype, and order the sidebar tree using it.

It should be introduced as an integer with an input/text, for people who haven't javascript enabled.

To make it easier, we should implement a drag&drop list to set a page position with the mouse.

[cash]

Storing trees in a relational database is not easy - or at least it is not easy to do well. We will need to plan this carefully.

[sembrestels]

I do not want involve the database in this. We have entities related by metadata, parent_guid. Now I want to introduce another metadata, priority.

We don't need to check if there is another page with the same priority (do not matters), and anything of that. Only save an integer, which is used to set the priority when showing a tree.