Automatic block formatting

[vkbo]

Epic #7

When hitting enter in the editor, the new block needs to set a reasonable block format. The default behaviour of the editor generally works well in that it copies the block format from the previous block, but some more controlled behaviour like in word processors would be nice.

• The next block after a header should be a paragraph block.
• If automatic first line indentation is enabled, this should be applied from the second block after a header or an empty block,