Use files with content-aware filenames as input

[TheAssassin]

AppImageLauncher introduced content-aware filenames (filenames that contain a hashsum of the actual file) to allow having multiple files with similar filenames but different contents in the integration directory (~/Applications by default).

libappimageupdate should make use of such files when updating AppImages, as there might be files with content-aware filenames around that might contain a good portion of the new file (if not all, if the only difference to the file on the server is the filename). This will greatly improve the efficiency of AppImageLauncher's updater, which uses libappimageupdate internally.