Implement setCursor handler for editable annotations

[etpinard]

Currently, hovering over editable: true annotations does not change the mouse cursor - making it hard for user to realise that editable annotations can be moved around.

See https://github.com/plotly/plotly.js/pull/660/files#diff-a123bebe752778fa4640d499ad73277cR375 for more info.

[rmoestl]

Has been implemented in #2594.