unify important stuff to colors

Right now we use tags in Photos, Bookmarks in Midori and Playlists in Noise.
If you look closely they all do the same, they group important stuff. Let's get rid of this and use color bullets (similar to Mac OS X).
+ The workflow would be similar across all apps so the user would benefit a lot.
+ E.g. Songs in a red "playlist", would also have a red label on pantheon-files.
+ There is a feature planned for pantheon-files, which will allow arranging items by colors, so with this overall change it will be even more useful.