An optional overlay button that appears near the bottom bar when in floating mode.
When set, this button will be positioned 18px away from the floating bottom bar. The positioning depends on the floating_position: - BOTTOM:
Button appears 18px to the right of the bar - LEFT: Button appears 18px above the bar - RIGHT: Button appears 18px above the bar
The overlay button is only visible when the BottomBar is in FLOATING mode. Setting this to null removes the overlay button..