Edit values by clicking

[ticalc-travis]

For editing node values, I think being able to click on the value directly to make it editable would be a convenient shortcut for having to click the edit button out to the side. The new/edit/delete buttons are kind of small and close together, and edit is right in the middle, so it's possible to click the wrong one by mistake. Since editing a value will probably be the most common operation used, being able to click the value itself to go straight into editing would make the mouse target larger and thus easier to hit, and I think it's also an intuitive action.