"Services" support

[ylt]

Allow plugins to expose a standardised service interface. This will allow similar features between many plugins to be handled the same.

For now will be returning a plain list, however a service repository class could be nice, but not sure of a tidy way of implementing this. This would allow doing i.e. manager.getServiceRepo("chat").sendChat("qwerty") to send a message to every chat service.
(maybe worthy of its own issue)

Plugins will list the services that they expose in the meta.json. Each service entry will specify a class within to use.

PluginManager: getService(serviceName), returns a list of services

meta.json ->

{
    "exposes": {
        "plug": null,
        "chat": "chat",
    }
}