Allow adding new images to the app

[niccokunzmann]

It should be easy to add new images to the app

• Images in the DCIM/coloring book folder
  • [ ] with a time stamp and images without should have a different order: First the images which are not like a time stamp and then the images which are like a time stamp. This way, templates added by the users are shown, too.
  • [x] There should be a documentation about this feature and the way you can add new templates.
• [x] allow sharing from browser
• [x] pre-compile a set of images from the open clip arts library
• create an installable app which can be started to choose even more images
  • propose to install this app using F-Droid/Google Play

[niccokunzmann]

time stamping is deprecated. The images are imported by sharing.