The idea of this PR is simple: We only have to worry about the position of the cursor when selecting a new block, everything else should be left as uncontrolled browser behavior.

Based on the above statement, this PR do:

• remove the focus prop from the block edit function and replace it with an isSelected prop
• no need for setFocus prop
• remove focus information from the state
• handle all explicit focus calls in the WritingFlow component: When a new block is selected, move the cursor to the right position
• I kept the focus and setFocus prop for the block's edit function for backwards compatibility concerns now.

This should simplify the block author's work a lot, not need to handle any focus behavior inside blocks.

Testing instructions

This needs heavy testing, all writing flow micro interactions should be tested.