Speed up creation of our `DataModule`

[bojobo]

Right now, we scan every directory for images and return a list containing the paths to the images. Since this can take some time, do the following:

• Check for a file containing a list of all paths
• If it does not exist -> Create a new one
• If the time of the creation of this file is older than the latest change to the directory -> Create a new one
  • I don't think that this will be needed, but it could avoid issues if files are removed/changed
• If there is a file -> Load it