Right now ImageWidget is kinda messy, it has a lot of if statements to say the least. I propose this to start off to simplify it. If someone needs something more complex they make their own widget.

• get rid of axes order kwarg, only allow a specific order, xy, txy, tzxy. It's upon the user to transpose dims appropriately.
• get rid of dim sliders kwarg
• allow rgba, maybe with a kwarg, and check that the last dim has shape 3.