Spotify > Dev Tools: short descriptions added.

Spotify Apps, Spotify Widgets, Libspotify SDK and Metadata API
Merge tag 'rev02-fix' into chapter2
chap3: draft stuctured updated.
rev03: Merge branches 'chapter1', 'chapter2' and 'chapter3'
chap3: Spotify apps.

figures added.
fonts removed.
chap 3: Spotify dev tools updated

added widgets figures.
added Metadata API subsubsection
chap 3: spotify > dev tools done.

experiments done.
chap 3: technologies updated.

Spotify desktop client added.
Merge branch 'chapter3'