#642 checkboxes for worksheet items

- press 'x' to check/unchecked focused item
- press 'd' to delete items (WIP, check console.log)
Delete items via checkboxes (WIP)
Add keyboard shortcuts and stubbed methods for inserting items and rows (WIP)
#628 add markdown items above or below current focused item

- Adds new empty markdown item, focuses on it, and sets it to edit mode
- TODO: if the user escapes from edit mode (without saving), the item still exists as an item but not as a component, which affects the parent's indices
Update delete method (WIP)
Refactor adding items and deleting items. Restore click-to-focus functionality
Merge pull request #655 from codalab/feature/560-delete-worksheet-items

Feature/560 delete worksheet items