Plugin System Update

[Berserker66]

As of now, many parts aren't updated to 1.3, and there is no selection of appropriate reader functions per world version (get_*)

Currently, these things are missing for a complete update of the plugin system:
reader functions for 1.3 formats beyond tiles; so missing is:
chests
signs
npcs

as well as database entries for new NPCs

Furthermore, a system for selecting the matching get_* functions needs to be implemented.

Additionally, all write methods need to be updated to 1.3. world generators currently use Terraria 1.1 format.