Item_factory: add support for loading items of type it_stationary.

- Simplify the constructor of it_stationary to make it usable in Item_factory (all the values of it_type are loaded _after_ the object has been created).
Export the flyer item to json.
Allow snippet texts to be read from an array

instead of having each entry in a separate json object.
Add documentation for json type STATIONARY and snippet.
Add extraction of translations for stationary items
Merge remote-tracking branch 'BevapDin/export-it_stationary-to-json' into FrankenMod

Conflicts:
	data/json/items/newspaper.json