Move savegame storage to a more reliable model:
* On loading, images are immediately imported into the local storage and get a UUID as name. (This fastens saving as the image does not have to be written to disk.)
* Savegames contain a reference to the used base image (so multiple savegames can use the same base image, thus reducing the storage size).
* When savegames are deleted, they are only unregistered. On every startup, a cleanup thread removes old images and configuration files.
* This concept simplifies the implementation of import and export feature (which will hopefully appear soon).