Change imgui submodule to docking branch

I've tested, the docking features, they seem to work as is, multi-window is optional, and to support it we'd have to implement some changes in the backend. (GLFW example implementation could possibly be useful for a port to OpenTK)

It might be wise to merge this to a docking branch of your own.

I've disable throwing Exceptions for notifications or low severity debug callbacks, this seems to be needed on my GPU driver even before these changes.