Move some common code from Draw:image implementations into lib-window

Move some common code from Draw:image implementations into lib-window

lib-window now provides Draw:scale-image which is used by Draw:image to
interpret the request and provide simple requests for the Drawing
engine.
pygtk doesn't use this yet.

Draw:scale-image should probably parse the file name etc too.

This fixes a bug in image display in ncurses where the image isn't
centred vertically when it isn't high enough to fill space.

Signed-off-by: NeilBrown <neil@brown.name>