Allow to choose the default scale

This adds a radio menu bound to a new setting "default-zoom", which applies to images displayed for the first time. To the current behavior called "smart", which remains the default, are added "fit-to-view" and "real-size".

If this setting is changed when an image list is already loaded, the scale is reset for all images. This is probably what is expected in general, and typically when switching from windowed to fullscreen mode, using "smart" or "real-size" for the former, and "fit-to-view" for the latter.