Add Custom Controls Icons on the existing toolbar

[jo-chemla]

Hi there! On current mapbox-gl-draw version with a custom control set-up, it is not straightforward to add to the mapbox-draw toolbar a button/icon that, when selected, triggers the drawMode change corresponding to that custom control.

I was able to setup a drawRectangle control with some help from miller here , and can set the defaultMode: 'draw_rectangle', to use it when map loads. But adding a custom button that calls my custom draw control can't be done right away.

Note: hiding the existing toolbar and adding a custom one like mentioned here seems to be the way to go at the moment