Images on demand

load images on demand... before they are displayed but not all initially:
so this loads initially the images from the current profile plus two profiles in both directions; on going forwards or backwards it always loads the next but one profile's image(s)... in theory this should be enough to always have the images prepared; in praxis we might want to add a loading spinner or at least a default background img or something indicating that the real content is still loading... what do you think @jeanggi90 ?

fixes #21 (closed)