Review how images are managed

[DamienCassou]

Currently, the Launcher searches for images in a particular directory and arranged in a special way: each image must be in a sub-directory with the same name as the image file. This has some drawbacks:

• if a user uses save as... in Pharo, the new image will not appear
• if a user has images downloaded without the launcher, the launcher won't see them unless the user renames and moves the image

It might be better to give the launcher a list of directories to recursively search for images.