Show gallery images in images' choice

[niccokunzmann]

When I choose images, I would like to see the gallery images listed automatically.

• [x] create settings activity
• [x] this is off by default (privacy)
• [x] add gallery images to the picture choice
• [x] allow to change the gallery url in the settings
• [x] allow http access to the gallery as a choice
• [x] add fields for custom urls to galleries
• [x] images from custom galleries must be shown first
• [x] save image previews in gallery
• [x] get small preview when possible
• [x] cache preview image
• [ ] images with the same name are not shown twice
• [x] load images in a thread

gallery:

• [x] add index (json) each image contains
  • the url (relative to gallery)
  • the preview url if existent
  • a link to the license
• [x] automatically generate preview images

[niccokunzmann]

https://github.com/niccokunzmann/coloring-book-gallery/commit/e5006011d5eddd53f14dacbe0de98ec3848cdf58 creates a JSON file with the content.

[niccokunzmann]

Now discussed in #103