Placeholders: addons should be able to register placeholders "on the behalf" of a gamemode

[Poslovitch]

Description

Is your feature request related to a problem?

Currently, addons register their placeholders "on their behalf". That means that placeholders from e.g. the Levels addon are prefixed with the addon's capitalized name %Level_[...]%. While this is okay with most addons, addons that have a bigger name (e.g. MagicCobblestoneGenerator) just turns this is into a stupid design issue.

Describe the solution you'd like us to implement.

These methods should be added in the PlaceholdersManager:

• #registerPlaceholder(Addon, GameModeAddon, String, PlaceholderReplacer)
• #unregisterPlaceholder(Addon, GameModeAddon, String)

We indeed need to provide the addon's instance nonetheless, so that we can keep track of which addon registered which placeholder.

This will require a slight adaptation in the code of the PAPI and MVdWPAPI hooks.

Describe alternatives you've considered.

None.

Additional context

None.

[Poslovitch]

This mostly relates to BentoBoxWorld/MagicCobblestoneGenerator#30.