Handle multiple wallpapers with same filename

[wdsrocha]

Currently, two wallpapers in two different directories cannot have the same filename.

Current behavior: only the first one will be converted to thumbnail.

Possible fix location: to_thumbnail function at make_gallery.py

Possible solutions:

• clone the wallpapers/ directory structure into thumbnails/ (preferred)
• include a counter on thumbnail filenames (not preferred, as it may add too much complexity when mapping the thumbnail file to the wallpaper file on README.md creation)

Other solutions are welcome.