Context buttons for saving and canceling label changes

[herongrove]

As a user, I want context buttons to appear when I'm editing a text field (such as a token of the sentence or a relation label). These buttons should be ✔ (tooltip: "Save") and ✘ (tooltip: "Cancel") or similar, and they should correspond to saving the edits I'm making or canceling my edit, respectively. Just like the Enter and Esc keys, the changes made this way should be reflected in the parsed data.