Right-click in the editor does not work in Firefox on Widows

(works fine on mac or in other browsers)

Apparently the `mousedown` event is not triggered during a right click (but `mouseup` and `click` events are), probably because the event is replaced with a `contextmenu` event that P:S listens to and prevents (see `src/js/addlisteners.js` and function `rightClickCanvas` in `src/js/inputoutput.js`).