Super simple:

Library.LibDraggable.draggify(window, callback)

makes window draggable and calls callback(window, x, y) every time the window is moved (in case you, say, want to SavedVariables the location.)

Does not save locations, just provides hooks to let you do it.

This is pretty trivial, but it's easy to get almost-right, and it's something I wanted more than once.