Cache images locally

[jsayol]

NW.js already has some sort of caching built in but it seems to be very unreliable. Every so often all the images get fetched again whenever they are displayed, which might become a problem if the user is on a slow/metered connection or just offline.

As the default behavior, instead of pointing to the tmdb site to show the images the app should cache them locally first and show that copy.

Additionally, add these in the settings:

• Option to disable the cache and just show the images "live".
• Option t empty (delete) the current cache. Ideally, it should let you choose either all of it or just that of a single collection.