Path fixes

- We no longer need to set PYTHONPATH
- We could run outside source dir
- If python uses the sources, then 'Running from source tree, using local files' will be printed

To test:
- Rebuild gnome-music
- Uninstall gnome-music
- Try running inside source dir (python3 ./gnome-music)
- Rebuild gnome-music
- Try running outside source dir (jhbuild run gnome-music)