Adds detection of backspace at start of editor block.
Inline break support.
Multiple fixes for edge cases.

Includes breaking out of formatting, establishing focus on newly created
block, and temporarily disables return removals.
Updates concatenated sources once more.
Merge branch 'fix/block-division-in-nested-nodes' into feature/block-join-shortcut
Enables joining of blocks via backspace.
Changes double enter blocks to always be text.
Merge branch 'fix/block-division-in-nested-nodes'
Merge branch 'feature/block-join-shortcut'