[Feature request] .move() the main window/widget in a cross-platform manner

[rubyFeedback]

In the event someone may want to add more features ... :)

If this is already possible then please excuse the feature request - I looked at
https://raw.githubusercontent.com/andlabs/libui/master/unix/window.c at the least
but could not find it. Perhaps it is implemented somewhere else, or missing.

I would like to have the ability to .move() on widgets. I am used to this on
ruby-gtk, in particular to position the window on the top left corner of the
screen consistently. (I am aware that alternative models exist such as
re-opening from the closed position, but I love starting from the top left
for my own widgets, so I would like to have this functionality as well.)

I believe this should work cross-platform since it is basic functionality,
but I'd be perfectly happy if it is first added to linux only. :P