Improved Docks

[BugelNiels]

Allow for the spawning of multiple copies of the same dock. In particular, allow for multiple image viewers and allow the viewer node(s) to select which dock to view the image in.

Concretely:

• Allow for (at runtime) adding/removing dock widgets
  • Docks should not have an enable/disable button anymore, but simply add/remove
  • Do something similar to blender where you have a dock wrapper and you can select from a dropdown which dock type to use.
• Each copy of a particular dock type gets an index (unique within said dock type)
  • This index should be displayed in the widget
• Viewer nodes should be able to specify the dock type in question