Add ability to unselect a radio button

[pingfan-hu]

Currently, if an mc or mc_buttons is clicked, it cannot be unchecked, unless the participant decides to select another radio button.

There are 2 possible solutions:

1. Add JS code to enable unchecking upon a 2nd click;
2. Add a button on the bottom right corner to clear selection.

I intend to try the 1st solution since it doesn't add anything in the UI. If it's not possible I'll move on to the 2nd solution.