[Feature] Multi layer onion skinning

[MrStevns]

This feature allows you to see onion skinning across all layers when enabled.

The use case is being able to draw separate entities of a character on several layers.
eg. you draw a man walking on layer A, and you want to attach a cape to that man without drawing on the same layer. While that was possible before, it was more difficult to do so because you would only be able to see the current frame of the man walking, while you might also need information about the previous and next keyframe.

Here you can see it in action. The head onion skin is on one layer and the hair onion skin is on another.

I have deliberately not added this to the camera painter because I don't see it having the same use case as drawings.

Also, while it could be argued that you would also want to be able to select which layer to skin specifically, that will have to be another feature request.

For now if you need to work with say only 2 layers, you can hide the other layers.

The feature can be disabled from the onion skin panel: