quick_frontend (constants, quick_app, quick_contact_list): blogging refactoring (not finished): - adaptation to backend modifications - moved common blogging parts from Libervia to quick frontend - QuickApp use a MB_HANDLE class variable to indicated if blogging is managed by the frontend (and avoid waste of resources if not) - comments are now managed inside parent entry, as a result a tree of comments is now possible - Entry.level indicate where in the tree we are (-1 mean parent QuickBlog, 0 mean main item, more means comment) - items + comments are requested in 1 shot, and RTDeferred are used to avoid blocking while waiting for them - in QuickBlog, id2entries allow to get Entry from it's item id, and node2entries allow to get Entry(ies) hosting a comments node - QuickBlog.new_message_target tell where a new message will be sent by default