- POTD was cached in ~/.cache/. But lock screen seems not a process of current user, so it won't find the cache in user's home dir. So I changed cache location to /tmp and it starts to work.
- Flickr and APOD(NASA) providers cannot cache pictures in PNG but JPEG works. So I simply changed the cache format to JPEG. Most POTD are photos. JPEG saves more spaces.
- Added a KDED module to cache lockscreen POTD, because lockscreen program doesn't have internet access.