Add selectedLeft and selectedRight properties

[vtslab]

The selecetedLeft and selectedRight properties would contain the indexes of available options that are selected/highlighted in the respective listbox before any of the buttons of the dash-dual-listbox is pushed. This allows to add additional buttons that allow the user to modify the content of the option labels, provided that the rendering of the dash-dual-listbox is simultaneously triggered.

React-duallist already exposes selectInLeft and selectInRight function properties that would allow to implement this in dash-dual-listbox. If my experiments prove succesful I will provide a PR for this.