Add color support for messages

This commit contains some dummy message functions (_type).
They are intended to be removed in a later commit
Overload message functions

… and get rid of those ugly “_type” things
Overload player.cpp wrappers around add_msg
Ensure new game_messages have always a type.
Change enum game_message_type
Put most game messages (add_msg …) into an appropriate category
Make new NPC wrapper functions around add_msg (with type)
Colorize melee battle messages
Colorize death and guilt messages
Colorize mutation messages
Colorize monster messages
Fix flipped message colors for mutation changes
Document “enum game_message_type” in ENUMS.md
Categorize messages related to inventory letters
Small fix regarding default game message type
Clean up redundant code
Add back a lost string
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.