make the stacking order of the 2D viewer images explicit

currently the order of stacking on the 2D canvas depends on the insert order in the `imagesToLoad` array
this is very implicit and could cause uncertainties about how the stacking of the images is

instead of having it implicit we should add somekind of "type" to the array objects (type could be "annotation", "grid" etc)
and then sort the array by type according to a ("type", order) list
something like
```
[ ("annotation", 1), ("grid", 2), etc ]
```
then it becomes very clear how the images are stacked (ordered)