Update Imgui cursor shape

[dimateos]

Imgui supports changing the platform cursor shape based on the context:

I feel like the resizing indicators help a lot when working with docked windows.

This would be platform dependant, but at least for windows it should be very similar to what is done in the official win32 backend:
https://github.com/ocornut/imgui/blob/master/backends/imgui_impl_win32.cpp#L757-L761