BlockToolbar and BalloonToolbar should init after all plugins are init-ed (and afterInit-ed).

[niegowski]

Suggested merge commit message (convention)

Fix (ckbox): Plugin order should not matter when it comes to registering schema for ckboxImageId attribute. Closes #15581.

Fix (ui): BlockToolbar and BalloonToolbar plugins order should not matter when it comes to registering toolbar items. Closes #15581.

MINOR BREAKING CHANGE (ui): The contents of the BlockToolbar and BalloonToolbar are now filled on the EditorUIReadyEvent instead of afterInit().

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Additional information

This PR solves actually 3 issues:

1. When adding the CKBox plugin before the  ImageBlock and ImageInline plugins, the schema for schema for ckboxImageId attribute was not registered.
2. Loading BlockToolbar (or BalloonToolbar) before CKBox (or actually any other plugin that registers toolbar buttons in afterInit()) resulted in the button not being registered.
3. Because the InsertImage integration sometimes happens in init(), and it is possible to load the plugin ui part before editing, it could happen that when the ui was loaded first, the command was not yet created, resulting in an error saying that it was not possible to bind dropdown's isEnabled to the observable (the command) - because the command was undefined. So we added the ability to pass a function to the integration registration instead, and we used that in all of our integrations. This way we always make sure that the command is there when we bind it.