Make onEnable always run before worlds load, remove onLoad, remove load: from plugin.yml

[dktapps]

Description

This is linked to the following issues: #3067

We want to get rid of onLoad, and make onEnable the defacto standard (or possibly the other way round).

Right now, the only reason to use onLoad is if you want to use load: POSTWORLD to have onEnable do things after the worlds are loaded within the same plugin. This is an uncommon case, and it's also clunky: you can't register event handlers or schedule tasks during onLoad. This case could be better served by adding an event, e.g. StartupWorldsLoadedEvent, so that plugins can run their onEnable before the worlds load and do logic at the POSTWORLD step.

In short, your options for startup work are as follows:

• POSTWORLD (most plugins) - you can't register new generators, world providers etc in time for worlds to load
• STARTUP - you can do some extra stuff pre-world, but you can't do anything to worlds
• POSTWORLD + onLoad - you can do some pre-world stuff and all normal post-world stuff, but you can't listen for world loading events, plugin load events, etc - onLoad also can't disable the plugin

With this proposal:

• onEnable would effectively always have STARTUP order - you'd be able to do everything except interact with worlds
• StartupWorldsLoadedEvent would allow you to do whatever extra postworld work you need
• No weird restrictions and shuffling code between onLoad and onEnable trying to get them to work

This also has the happy side effect of deleting a confusing plugin.yml directive, making onEnable behaviour more predictable (no need to look at plugin.yml to see what you can and can't do), and removing some API cruft (no more getting confused between onLoad and onEnable).

Alternative methods

We could just remove the restrictions from onLoad instead (and perhaps make onEnable always run at POSTWORLD). However, I think that this would make the intended purpose of these two hooks even more confusing, as onEnable wouldn't live up to its name anymore.