Add SPDX info to all files

Move caching to src/cache/; Finalize API parsing fixes

Font and user cache code has been moved to special files, independant from src/config.rs

API parsing changes have been properly tested, and last.fm API JSON is now trace-logged for debugging convenience.

Reliable user info; Font refactor

Hit a few more endpoints to fix missing images, fallback to default album art just in case.

Refactor the font cache into its own file.

Custom font support complete

Add custom font support.