Store position in window and provide an API to recover it

[mwestphal]

Is your feature request related to a problem? Please describe.
Because of https://gitlab.kitware.com/vtk/vtk/-/work_items/20112, we cant rely on VTK to store the position.
Also Cocoa has some specificities on that regard.

In order to provide a getPosition API and also to be able to load/save statefiles containing the position, we need to store it internally

Describe the solution you'd like

• In window_impl.h, store the position as members
• Add a window::getPosition() API and bindings

[mwestphal]

@Meakk wdyt ?

[Meakk]

That's a shame VTK cannot handle it but the workaround doesn't seem that bad.

[snoyer]

VTK MR hopefully resolving the linked VTK issue here: https://gitlab.kitware.com/vtk/vtk/-/merge_requests/13472