Basic editing of todos

[ChrisWay]

This pull request is to track the basic editing features need for a 0.1 release. Editing will be done inline, the row in the ListView will change to a text box.

• Save edits that have been made to existing lines
• Save new lines
• Clicking on a row should enter edit mode
• Pressing 'E' with a row selected should enter edit mode for that row
• Pressing Esc should revert any changes and leave edit mode
• Press up/down arrows should leave edit mode and move selection appropriately