Introduce "mouseTouch" option to avoid dragging with the mouse when "touch" is enabled

The `touch` option works great on touch devices, but it also allows dragging slides with the mouse which I found very confusing (e.g. makes it impossible to select text inside a slide).

So I introduced a new `mouseTouch` option to enable mouse dragging explicitly. It's set to `false` by default now since that makes more sense to me.

What do you think? :)