Make events consistent by introducing data.ready and plugins.ready

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Extracted from #1449.

In #1449 the editor.uiReady event was moved from Editor to EditorUI and renamed to ready which means editor.uiReady was replaced with editor.ui.ready event.

To improve the API and keep it consistent, same could be done with dataReady and pluginsReady events (first is fired by specific editor creators and second by the Editor instance).

The dataReady should become editor.data.ready (fired by DataController class) and pluginsReady should become editor.plugins.ready (fired by PluginCollection class).

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When it comes to dataReady event and moving it to DataController class - it should be fired on the end of the init method (somewhere here probably). Now, there is another event already used in DataController called init created by decorating the init() method.

This means there will be two events - init and ready fired in the exact same moment (btw. ready could be also implemented using decoratedelegate). IMHO it doesn't make sense, because these two events just indicates the same thing (data initialization has finished). Should we get rid of init event?

The second thing, is it possible to have editor creator which does not use data.init method (such create() method is used in Editor class)? In such cases, data#ready event will not be fired. Should it be mandatory to use data.init() method or fire data#ready event manually when implementing editor creator (maybe it already is)?

cc @pjasiun @Reinmar

[pjasiun]

When it comes to dataReady event and moving it to DataController class - it should be fired on the end of the init method (somewhere here probably). Now, there is another event already used in DataController called init created by decorating the init() method.

init event is needed to make it possible to overwrite this method. However, we could fire ready in the init listener with the LOWEST priority. It makes sense that at the end of the initialization data controller says it is ready. It would also work with custom listeners which adds additional functionality to the init method.

The second thing, is it possible to have editor creator which does not use data.init method (such create() method is used in Editor class)?

I think each editor should init engine using data.init. We should remove this Editor.create method since it is simply incorrect.