drag-and-drop mechanism

[xarthurx]

Hi, I would like to have a "drap-n-drop to open file" feature and was wondering if this is sth that should be embedded into the viewer.

On ImGui's page, I found this: https://github.com/ocornut/imgui/issues/2602

But I'm not sure how this should be integrated into polyscope to be multi-platform compatibable.