Adds modifyBlocks option to make structure read-only

[hermzz]

Setting the option to false won't allow users to add/remove or re-order
blocks.

[raffij]

Thanks for this PR @hermzz. It's a good idea and something we'll happily merge in.

So you know. One of the things that hasn't been fully resolved since we moved from a block to a document based approach is the block limitations like this.

Something you might not have considered is pasting content into a block. Try pasting a couple of paragraphs of text into a text block and see how it behaves and it's as you expect.

I'm currently working on improving block selection which includes some paste improvements. So it might be best to wait till those are done before making sure the above PR does as you expect too.