Allow adding custom unlocks and linking them to items or characters

[DarkestSentinel]

Especially for bigger mods it's nice to have unlocks so players still have something to work towards. The achievements.xml file could be used for this, granted modders can access those unlocks through lua and calling Unlock() on them when certain conditions set by the modder are met. The unlock paper would show with the .png set in the achievements.xml. Giving the option to link the unlock to items and characters would be very convenient.

More functionality like being able to do this with any existing achievements in the game, or having the ability to also lock unlocks again would also be nice to have.