Apply bidi to the root block by default

[ahangarha]

In many cases, the content under a block follows the same direction of the root block.

Applying the bidi to the root block has these advantages:

• Better UX when adding sub-blocks in different direction (in most of the cases)
• Better performance because of less processing

We can add an option in the setting for applying bidi to the whole tree. Maybe we can call it greedy.