allow subwindows to be moved with mouse

[dankamongmen]

Now that we're using Notcurses, it ought be pretty trivial to allow the user to move subwindows around on the screen with their mouse. The position to which a window is moved ought probably be remembered, and used again if the window is toggled off and on. Add some manner of indication that the window can be moved (and probably allow it to be closed with the mouse, too).