Fix the multiple list block shortcuts

[youknowriad]

I noticed two things:

• List indent/outdent shortcuts don't work properly when you have multiple list blocks (callbacks are triggered in all list blocks while they should only affect the selected blocks)
• Also this has some impact on performance since keydown handlers number grow significantly as we add list blocks to the post.

This PR wraps those shortcuts in an isSelected check to fix that.

That said, I also took a look at the RichtextShortcut component and after some thinking I concluded that this component is useless and also misleading.

• I wonder whether I should wrap the component automatically in ifBlockEditSelected like block controls... but that's not a good approach because conceptually speaking, you can have a RichText outside a block or multiple RichText in the same block, so this won't solve anything really.
• It gives you the impression that it tied to RichText somehow but in reality, it's not it's just a proxy to the KeyboardShortcuts component.
• I think ideally, we should just deprecate that component. I didn't do that yet.